Add CLA Agreement Tracker integration

[pellared]

GDI repositories should be integrated with https://github.com/splunk/cla-agreement

We should also demonstrate how to do this. It can be even done in this repository and reused as a composite action.

[breedx-splk]

Do we have any other reasonable way right now to verify that external contributors have signed a CLA? Are any signalfx repos doing this yet?

[pellared]

Do we have any other reasonable way right now to verify that external contributors have signed a CLA?

Not any that I am aware of.

Are any signalfx repos doing this yet?

Not yet. Proposing here #268.
PS. This is recommended by our legals (and admins of https://github.com/splunk).