Implement Cosmos AAD authentication

[johnbatty]

Add support for Cosmos AAD authentication via TokenCredential:

• Extended AuthorizationToken enum to add TokenCredential variant
• Added new AuthorizationToken::from_token_credential(...) constructor
• Removed PartialEq and Eq implementations from AuthorizationToken, as it is difficult to compare dyn TokenCredential instances, and I'm going to assert that users probably don't need to compare AuthorizationTokens.
• Updated generate_authorization(...) to be async and return a Result<...>, as it now has to be able to call token_credential.get_token(...)
• Did not implement TokenCredential support for PermissionTokens, as it doesn't seem to make sense to support AAD for these.

Added new example document_entries_aad.rs to demonstrate and test the new function (I've tested it and it works).

Fixes: #1230