Add support for more authentication methods

[sdebruyn]

I've been struggling with the current authentication methods and they always require a little bit of configuration effort while authentication with other azure-identity-based packages usually works quite easily.

So I've added 3 new ways to authenticate to your Azure databases:

• Automatic (really the easiest way)
• By reading environment variables
• Managed identity

All methods are properly documented on https://docs.microsoft.com/en-us/python/api/overview/azure/identity-readme?view=azure-python but I've updated the README as well.

I added tests for everything except managed identity since that is impossible to test without an actual managed identity configured.

I removed the storage of client ID & secret in the service principal authentication as environment variables to avoid side effects from using DefaultAzureCredential and to avoid leaking credentials outside of this context.

[JCZuurmond]

LGTM

I see you ran black, which is fine with me. Maybe @swanderz we should add this in a pre-commit hook and test for it in the pipeline see #72

[dataders]

@sdebruyn this looks great! I especially love the tests you added and the documentation. happy to merge and release a new version with this change ASAP. can you please first:

• increment the version in dbt/adapters/sqlserver/__version__.py to 0.21.1?
• add an entry for v0.21.1 to CHANGELOG.md?

[sdebruyn]

@swanderz done :)

[dataders]

thanks @sdebruyn!!!