(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/common/asyncIterators.ts

@@ -0,0 +1,25 @@
+/**
+ * Just some basic utilities for async generators that should really be part of the language or lodash or something.
+ */
+
+export async function* asyncFilter<T>(it: AsyncIterableIterator<T>, pred: (x: T) => boolean): AsyncIterableIterator<T> {
+  for await (const x of it) {
+    if (pred(x)) {
+      yield x;
+    }
+  }
+}
+
+export async function* asyncMap<T, R>(it: AsyncIterableIterator<T>, mapper: (x: T) => R): AsyncIterableIterator<R> {
+  for await (const x of it) {
+    yield mapper(x);
+  }
+}
+
+export async function toArray<T>(it: AsyncIterableIterator<T>): Promise<T[]> {
+  const result = [];
+  for await (const x of it) {
+    result.push(x);
+  }
+  return result;
+}