Updated Azure storage compatibility

[matthewhilton]

Changes

• Adds new azure_blob_storage_file_system class to replace the deprecated azure_file_storage_system.
  • This is intentionally added as an additional class (as opposed to renaming/fixing the old class) because the new class requires a new plugin dependency https://github.com/catalyst/moodle-local_azureblobstorage
  • Having a new class allows time for admins to install the new plugin in parallel, and then switch over rather than suddenly finding the newest minor version update changed dependencies.
• Added documentation about migration process for the above
• Fixes a tiny bug in a test that assumed no files existed before the test
• Fixed a bug where testing deletion does not check if the fs even is allowed to delete, related to $testdelete setting in client does not respect plugin config, delete permissions checks may be outdated #645 but does not completely resolve the issue for other fs's like AWS

Testing

• The new client class with the new sdk plugin pass the integration tests (the standard objectfs unit tests but attached to azure storage via the azure_blob_storage_file_system)
• Also tested locally the various CRUD operations including large files uploaded via multipart upload and large files streamed down (e.g. videos) in multipart - all working as expected

[Peterburnett]

Should we be emitting debugging() anywhere when the old SDK is used? It has the potentially to be EXTREMELY noisy, just wondering if it would be useful with out drowning out noise.

[matthewhilton]

Should we be emitting debugging() anywhere when the old SDK is used? It has the potentially to be EXTREMELY noisy, just wondering if it would be useful with out drowning out noise.

The old SDK already emits enough debugging messages on its own because of the php deprecation notices. I don't think we need to make it any worse and believe people will have enough reason to switch over when it becomes apparent the old one simply doesn't work anymore.