mqtt.server: Add support for auth system

[balloob]

Description:

Add support to mqtt.server to use Home Assistant authentication.

Related issue (if applicable): #15703

Example entry for configuration.yaml (if applicable):

mqtt:

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

Going to close this as it requires us creating a user and configuring that, making more complicated than I planned. The solution by @awarecan in #15929 is enough.