Add health checks for testing readiness to serve clients

[the-mikedavis]

Proposed Changes

This covers the remaining two checks in #13782. One tests the connection limit for the node and the other is a kind of composite check that combines #13872 and checks whether listeners are available for AMQP/AMQPS.

Closes #13782

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

Put an x in the boxes that apply.
You can also fill these out after creating the PR.
If you're unsure about any of them, don't hesitate to ask on the mailing list.
We're here to help!
This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website (Document health checks for testing readiness to serve clients rabbitmq-website#2264)
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

These checks only apply to AMQP + AMQPS because that's what the connection_max currently applies to. In the future these checks could be updated to also cover other protocols. If desired I could pick up #9876. We can use ranch:info/1 as Loic suggested in #7593 to efficiently count connections per protocol.