ci: new workflow to notify TSC members in slack

[derberg]

This workflow throws a message in a dedicated channel when there is a mention of the GitHub team that groups all technical steering committee members:

• new discussion, issue or PR that mentions the team
• new or edited comment in issue or PR that mentions the team (message links back to the comment)
• new comment in discussion and new comment/reply to a comment in discussion (message links back to the comment)
• reopened issue or PR that mentions the team

Because of if condition in the job, and not steps, workflow is skipped if TSC members are not mentioned

I tested this workflow in new channel 95_bot-tsc-members-mentioned. Below you can see example messages that were triggered from my test repo.

See also asyncapi/community#253
This PR provides notifications to slack only for mentions.

Once we have a voting process facilitated, and we know how to do it, we will need to extend this workflow with new job that triggers a slightly different message, @channel with call for voting

[KhudaDad414]

LGTM!

[derberg]

@KhudaDad414 thanks! I'll merge it later today as the other workflow update is in progress and it will take probably an hour to replicate

[derberg]

/rtm