Added multiple adapters capability to adapter manager

refs https://linear.app/tryghost/issue/CORE-1/multiple-adapters-per-type

- There's a need to support multiple adapter variations per given adapter type (storage, sso, etc.)
- With the introduced changes we can specify a version of an adapter that should be fetched based on `:feature` postfix. For example:

`adapterManager.getAdapter('storage')` -  would return the default adapter listed under "active" configuration
`adapterManager.getAdapter('storage:videos') - would return an adapter configured for videos *feature*

- Here's an example configuration for a custom video storage:
```
"storage": {
    "active": "LocalFileStorage",
    "videos": "ghost-storage-custom-video",
    "ghost-storage-custom-video": {
        "custom": "configHere"
    }
}
```

core/server/services/adapter-manager/index.js

@@ -1,5 +1,6 @@
 const AdapterManager = require('@tryghost/adapter-manager');
 const getAdapterServiceConfig = require('./config');
+const resolveAdapterOptions = require('./options-resolver');
 const config = require('../../../shared/config');
 
 const adapterManager = new AdapterManager({
@@ -16,13 +17,16 @@ adapterManager.registerAdapter('scheduling', require('../../adapters/scheduling/
 adapterManager.registerAdapter('sso', require('../../adapters/sso/Base'));
 
 module.exports = {
-    getAdapter(adapterType) {
+    /**
+     *
+     * @param {String} name - one of 'storage', 'scheduling', 'sso' etc. Or can contain a "resource" extension like "storage:image"
+     * @returns {Object} instance of an adapter
+     */
+    getAdapter(name) {
         const adapterServiceConfig = getAdapterServiceConfig(config);
 
-        const adapterSettings = adapterServiceConfig[adapterType];
+        const {adapterType, adapterName, adapterConfig} = resolveAdapterOptions(name, adapterServiceConfig);
-        const activeAdapter = adapterSettings.active;
-        const activeAdapterConfig = adapterSettings[activeAdapter];
 
-        return adapterManager.getAdapter(adapterType, activeAdapter, activeAdapterConfig);
+        return adapterManager.getAdapter(adapterType, adapterName, adapterConfig);
     }
 };

core/server/services/adapter-manager/options-resolver.js

@@ -0,0 +1,18 @@
+module.exports = function resolveAdapterOptions(name, adapterServiceConfig) {
+    const [adapterType, feature] = name.split(':');
+    const adapterSettings = adapterServiceConfig[adapterType];
+
+    let adapterName;
+    let adapterConfig;
+
+    // CASE: load resource-specific adapter when there is an adapter feature name specified as well as custom feature config
+    if (feature && adapterSettings[feature] && adapterSettings[adapterSettings[feature]]) {
+        adapterName = adapterSettings[feature];
+        adapterConfig = adapterSettings[adapterName];
+    } else {
+        adapterName = adapterSettings.active;
+        adapterConfig = adapterSettings[adapterName];
+    }
+
+    return {adapterType, adapterName, adapterConfig};
+};

test/unit/server/services/adapter-manager/options-resolver.test.js

@@ -0,0 +1,74 @@
+const should = require('should');
+
+const resolveAdapterOptions = require('../../../../../core/server/services/adapter-manager/options-resolver');
+
+describe('Adapter Manager: options resolver', function () {
+    it('returns default adapter configuration', function () {
+        const name = 'storage';
+        const adapterServiceConfig = {
+            storage: {
+                active: 'cloud-storage',
+                'cloud-storage': {
+                    custom: 'configValue'
+                }
+            }
+        };
+
+        const {adapterType, adapterName, adapterConfig} = resolveAdapterOptions(name, adapterServiceConfig);
+
+        adapterType.should.equal('storage');
+        adapterName.should.equal('cloud-storage');
+        adapterConfig.should.deepEqual({
+            custom: 'configValue'
+        });
+    });
+
+    it('returns adapter configuration based on specified feature', function () {
+        const name = 'storage:videos';
+        const adapterServiceConfig = {
+            storage: {
+                active: 'cloud-storage',
+                videos: 'local-storage',
+                'cloud-storage': {
+                    custom: 'configValue'
+                },
+                'local-storage': {
+                    custom: 'localStorageConfig'
+                }
+            }
+        };
+
+        const {adapterType, adapterName, adapterConfig} = resolveAdapterOptions(name, adapterServiceConfig);
+
+        adapterType.should.equal('storage');
+        adapterName.should.equal('local-storage');
+        adapterConfig.should.deepEqual({
+            custom: 'localStorageConfig'
+        });
+    });
+
+    it('returns active configuration if piece of feature adapter is missing', function () {
+        const name = 'storage:videos';
+        const adapterServiceConfig = {
+            storage: {
+                active: 'cloud-storage',
+                videos: 'local-storage',
+                'cloud-storage': {
+                    custom: 'configValue'
+                }
+                // when you forget to configure local-storage!
+                // 'local-storage': {
+                //     custom: 'localStorageConfig'
+                // }
+            }
+        };
+
+        const {adapterType, adapterName, adapterConfig} = resolveAdapterOptions(name, adapterServiceConfig);
+
+        adapterType.should.equal('storage');
+        adapterName.should.equal('cloud-storage');
+        adapterConfig.should.deepEqual({
+            custom: 'configValue'
+        });
+    });
+});