add slack notification for RFCs

[rscohn2]

Description

Add support for posting notifications in UXL slack channel when a PR is labelled with RFC.

To test this workflow, you need to merge it to main and then open a PR, and label it RFC. GitHub will execute the
workflow file from the main branch. Since the workflow is coming from main, it will have access to the secret SLACK_BOT_TOKEN Vadim has confirmed that he added the secret to the repo.

Checklist

General

• [ n/a] Do all unit and benchdnn tests (make test and make test_benchdnn_*) pass locally for each commit?
• [ n/a] Have you formatted the code using clang-format?

Performance improvements

• [ n/a] Have you submitted performance data that demonstrates performance improvements?

New features

• [ no] Have you published an RFC for the new feature?
• Was the RFC approved?
• Have you added relevant tests?

Bug fixes

• [n/a ] Have you included information on how to reproduce the issue (either in a github issue or in this PR)?
• Have you added relevant regression tests?

RFC PR

• Does RFC document follow the template?
• Have you added a link to the rendered document?

@karturov @vpirogov

[vpirogov]

@rscohn2, please test the changes in a fork to confirm that the action works as expected.

Please also add a license header reflecting correct copyright.

[rscohn2]

@rscohn2, please test the changes in a fork to confirm that the action works as expected.

Please also add a license header reflecting correct copyright.

I copied the copyright from the other workflow file.

To test, I added SLACK_BOT_TOKEN to my fork and changed the channel to #rc-test in the UXL slack space. I merged the PR into the main branch of the fork. Then I submitted a PR that was labeled RFC, and a 2nd where I did the label after the PR was submitted. They both generated messages. If you look at the #rc-test channel, you can see what the message will look like.

[vpirogov]

Thanks, @rscohn2!

The only thing remaining is squashing commits and updating the commit message to something like 'github: workflows: add slack notification for RFCs'. Once this is done I'll merge the PR.

[rscohn2]

The only thing remaining is squashing commits and updating the commit message to something like 'github: workflows: add slack notification for RFCs'. Once this is done I'll merge the PR.

Done.