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
