(core) Add AzureExternalStorage

Summary:
Adds a new implementation of the interface ExternalStorage that works for Azure Blob Storage as an alternative to S3, for a specific self-hosting case.

Tweaks HostedStorageManager and ICreate to allow configuring different core implementations of ExternalStorage.

Followup tasks:

- Make this code available to self hosters, possibly by making it open source.
- Add an env var or other config option to specify the preferred type of storage. Currently using the var `AZURE_STORAGE_CONNECTION_STRING` to know how to connect to Azure when requested, but that choice still only lives in test code.

Test Plan: Generalized HostedStorageManager and ExternalStorage tests to test the new AzureExternalStorage alongside S3ExternalStorage. The HostedStorageManager tests also now test the 'cached' in-memory test storage in a way that's closer to the real storage methods.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3413

app/server/lib/ICreate.ts

@@ -20,7 +20,14 @@ export interface ICreate {
   //  - meta. This store need not be versioned, and can be eventually consistent.
   // For test purposes an extra prefix may be supplied.  Stores with different prefixes
   // should not interfere with each other.
-  ExternalStorage(purpose: 'doc' | 'meta', testExtraPrefix: string): ExternalStorage|undefined;
+  // innerCreate should be a function returning the core ExternalStorage implementation,
+  // which this method may wrap in additional layer(s) of ExternalStorage.
+  // Uses S3 by default in hosted Grist.
+  ExternalStorage(
+    purpose: 'doc' | 'meta',
+    testExtraPrefix: string,
+    innerCreate?: (bucket: string) => ExternalStorage
+  ): ExternalStorage | undefined;
 
   ActiveDoc(docManager: DocManager, docName: string, options: ICreateActiveDocOptions): ActiveDoc;
   NSandbox(options: ISandboxCreationOptions): ISandbox;