Updated mail_room and enable sentinel support to Reply by Email

What does this MR do?

• Updates mail_room gem to include Sentinel Support ( Issue / Pull Request )
• Added sentinel helper methods to Gitlab::Redis to support retrieving configurations for mail_room
• mail_room.yml includes sentinels listing when available

Are there points in the code the reviewer needs to double check?

Sentinels is tricky to test, and mail_room is even trickier. Locally I've used this gem to help simulate an IMAP server: git://github.com/y10k/rims.git

To execute it locally:

Add it to the Gemfile and `bundle install``

# to execute
bundle exec rims server -u 'user1@domain.tld' -w 'mypassword' -d fakeimap/

# to append mail (get a "full source email file" from a desktop mail client, and dump to a local file):
bundle exec rims imap-append -v -n 127.0.0.1 -o 1430 -w 'mypassword' -u 'user1@domain.tld' fakeimap/mail.txt

Why was this MR needed?

This is part of the effort to enable full Sentinel support in GitLab : #3355 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#3355 (closed)

Closes #21645 (closed)