Microsoft SQL Server (MSSQL) scaler implementation

[cgillum]

This PR adds a built-in scaler that uses a Microsoft SQL Server (MSSQL) database as the event source. It's very similar to both the existing mysql and postgresql scalers and is intended to work with both self-hosted SQL Server databases and SQL Server databases hosted in managed clouds.

Documentation PR: kedacore/keda-docs#367

There is still more work to do on this PR, which is tracked in the checklist below. This also includes end-to-end testing. However, I wanted to open it early to hopefully get some early feedback since this is both my first contribution to KEDA and my first time writing code in Go (besides hello world). Feedback is greatly appreciated. 🙏🏽

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• Unit tests have been added
• End-to-end tests have been added
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified) (doesn't seem like this is necessary)
• A PR is opened to update the documentation on (repo) (if applicable)
• Changelog has been updated

Fixes #674

[tomkerkhove]

Thanks for bringing this upstream @cgillum!

Can you please:

• Update our changelog (https://github.com/kedacore/keda/blob/main/CHANGELOG.md)
• Open a PR for our docs (https://github.com/kedacore/keda-docs)

Thanks!

[tomkerkhove]

@cgillum Can we mark the PR as ready for review or is there any open work?

[cgillum]

@tomkerkhove still working on putting together end-to-end tests for this one. I'm learning as I go (first time deploying a real k8s cluster), so apologies for the delay.

[tomkerkhove]

Oh no worries at all! I was just checking if you were waiting on us or not!

[lgmorand]

@cgillum can't help you but you have my moral support to finish your e2e test :) this scaler would be great to have

[cgillum]

@tomkerkhove and @ahmelsayed, I was able to complete my e2e testing successfully and I think this PR is ready to go. Would it be okay to defer the authoring of the e2e test for another PR or should that also be part of this PR?

[ahmelsayed]

@cgillum yes that's fine, e2e test can come in a separate PR

[cgillum]

I ended up getting an end-to-end test working sooner than I thought, so I went ahead and added it to this PR in my latest iteration. It's basically the same as the mysql test with a few tweaks and simplifications. Please let me know if the way that it is written is acceptable, especially regarding the dependency on my docker hub test container image.

[tomkerkhove]

If you have a test container, we could host it on our test container repo if you want?

[cgillum]

It probably makes more sense to eventually host it in a KEDA-specific container repo so that I don't accidentally break anything in the future. That said, this doesn't have to happen right away. We could move it as part of a separate PR once we know that the nightly tests are stable.

[tomkerkhove]

Sure thing, no worries! Will wait for @ahmelsayed his review.

Thanks!