Add test coverage github action

[fridgepoet]

This github action does the following for pull requests:

• Checks out this PR's code
• Checks out code on main
• Calculates test coverage for each
• Leaves a comment on the PR with a test coverage report with the difference between PR coverage vs main coverage
• When commits are added to the PR, the existing comment should be updated with an up-to-date coverage report.

To do:

• Backend coverage
• Add frontend coverage
• Migrate this action to another repo so it can be referenced from other repos

I was very much inspired by the guidelines defined by the Loki team but didn't use drone reports as was done in the proposed solution...

Contributes to https://github.com/grafana/cloud-data-sources/issues/5

[github-actions]

Code coverage report for PR #151

	Go	Typescript
main	64.4%	47.49%
PR	64.4%	47.49%
difference	0%	0%

[andresmgot]

Great! Looking good so far :)

[andresmgot]

LGTM! Do you plan to merge this before moving this to a separate package?

[fridgepoet]

Thanks! Once I've put the final touches, I think we can merge it as-is so that it can start seeing some action (::crickets::) and I'll move it separately afterwards.