auto start and stop Synapse db; streamline auth

[dataders]

Description & motivation

• Azure benefit subscription credits are running low. Have to cut costs ASAP. the new approach is that the Synapse pool will only be turned on when CI runs are triggered, then turned off afterwards
• Also standardizing on using a Azure service principal credentials to authenticate rather than the SQL Server username and password.

@jtcohen6 I'll pass you the new envvar set elsewhere.

Checklist

• [ ] I have verified that these changes work locally
• [ ] I have updated the README.md (if applicable)
• [ ] I have added an integration test for my fix/feature (if applicable)

[jtcohen6]

Looks good by me!

1. Pass along the new env vars when you get a chance, and
2. I need to figure out why the integration tests are failing with v0.19.1 in CircleCI, when they don't fail for me locally.

[dataders]

Tried out the new env vars, works like a charm! Thanks @swanderz!

sweet. that'll save us ~$950/month to only unpause the Synapse pool at time of PR instead of having it on 24/7. If only I could go back in time 3 months to make this PR....