Add HomeKit firmware revision characteristic

[schmittx]

Description:

Adds characteristic to accessory information service for firmware revision. By default, firmware revision is set to current Home Assistant version.

Related issue (if applicable): N/A

Pull request in home-assistant.github.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

N/A

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 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.

[cdce8p]

I'm not really sure this is necessary, but we can certainly add it.
Can you combine this PR into #14114?

[cdce8p]

Use the following instead:

    service.get_characteristic(CHAR_FIRMWARE_REVISION) \
        .set_value(firmware_revision)

[schmittx]

Done.

[schmittx]

I agree, not exactly necessary but I though it worked nicely to use the Home Assistant version. I've added this to #14114.