feat: beacon

[edgurgel]

What kind of change does this PR introduce?

Create new process group library called Beacon that broadcast only group counts to other nodes. Actual pids are only available to the local node. It also supports custom adapter so that we can use PubSub for the broadcasting (including regional broadcasting).

The plan is to start using Beacon without any real changes and later on start using the group data for 3 things:

• Connected stats
• Max connected users rate limit
• Connect & PG Changes check for online users for a tenant

TODO:

• Write a proper README.md
• Improve on docs
• Add CI step to test Beacon

[coveralls]

coverage: 88.124% (+0.1%) from 88.016%
when pulling 36c3250 on feat/beacon
into 05df771 on main.

[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
Realtime.UsersCounterTest: failure on setup_all callback, all tests have been invalidat ed › ** (MatchError) no match of right hand side value: {:error, {{:EXIT, {:boot_failed , {:exit_status, 1}}}, {:child, :undefined, {:peer, :main}, {:peer, :start_link, [%{nam e: :main, host: ~c"127.0.0.1", connection: :standard_io, longnames: true}]}, :permanent , false, 5000, :worker, [:peer]}}}/stacktrace	View Logs