Add availability_template to Template Switch platform

[grillp]

Breaking Change:

Not a Breaking change

Description:

This PR adds an availability_template option to Template Switch platform.

By defining the availability_template the availability of a Template Switch can be controlled using a template.

As Template Components rely on other component states, (i.e. they do not have state themselves) it would be useful to also be able to control when a component is unavailable. e.g. when a component that the template component relies on to function becomes 'unavailable' it would be useful to be able to mark the template component as 'unavailable' also.

If the availability_template is not configured, the device will always be 'available' (default behavior)

If the availability_template is configured, the device will be 'available' if the template returns true or 'unavailable' if the template returns any other value.

home-assistant/home-assistant.io#10310

Example entry for configuration.yaml (if applicable):

For a Template Switch platform component that requires a 'switch.some_sensor' to be available to operate:

switch:
  - platform: template
    switches:
      my_switch:
        availability_template: >-
          {%- if not is_state("switch.some_sensor", "unavailable") %}
            true
          {%- endif %}

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

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

• Documentation added/updated in home-assistant.io - Added 'availability_template' to all Template Comp home-assistant.io#10123

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

• [N/A] The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• [N/A] New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• [N/A] Untested files have been added to .coveragerc.

If the code does not interact with devices:

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