Bring in AAD auth from dbt-synapse?

[dataders]

@JosephCDonaldson mentioned they'd like AAD auth available as part of dbt-sqlserver. Perhaps I open a PR to try and merge our connections.py?

[mikaelene]

That would be great as long as the other auths work. Merge it in 0.18 if you find that it works. I’ll try to release it when dbt 0.18 is out.

[dataders]

We just realized we're missing one key AAD auth method, service principal. We'll aim to have a PR in by the end of the week with the 3 supported AAD methods plus service principal by the end of the week.

[mikaelene]

This is solved by PR #53 and #55