slack notification when PR is labeled RFC

[rscohn2]

Description

Post notification to onetbb channel in uxlfoundation slack space when a PR is labeled with RFC. To see a sample notification, look at #rc-test channel in uxlfoundation slack workspace. This is same as uxlfoundation/oneMath#481, except the channel name and making the license consistent with ci.yml.

To be functional, the PR must be merged and someone with admin access to the repo must add SLACK_BOT_TOKEN secret. I can provide the token in DM.

Partially addresses UXL open source best practices (don't see TBB in https://github.com/orgs/uxlfoundation/projects/5?)
@vbm23

Fixes # - issue number(s) if exists

• [ x] - git commit message contains an appropriate signed-off-by string _(see [CONTRIBUTING.md]

Type of change

Choose one or multiple, leave empty if none of the other choices apply

Add a respective label(s) to PR if you have permissions

• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• [x ] infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• [ x] not needed

Documentation

• updated in # - add PR number
• needs to be updated
• [ x] not needed

Breaks backward compatibility

• Yes
• [x ] No
• Unknown

Notify the following users

List users with @ to send notifications

Other information