Add MQTT notify platform

[jbouwh]

Proposed change

Add new MQTT platform notify.
The platform implements a MQTT notify service that supports setup from configuration.yaml and auto discovery.
The platform does not generate entities but does support a device payload to be sent in auto discovery mode.
The device will be created at discovery and will be automatically be removed if the device is unregistered with an empty discovery payload. If the device is removed from the device registry it will also remove the registered notify services.

A device can support displaying notification message through a LCD or OLED screen. The notify service can be discovered automatically and message can be send from automation scripts.

The current MQTT notify documentation page is also updated.

Some real-time other applications are:
https://github.com/jpmens/mqttwarn
https://support.kaspersky.com/IoTSecureGateway/2.0/en-US/193697.htm
https://github.com/maxbachmann/mqtt-mm2-gateway a gateway to https://github.com/MichMich/MagicMirror

MQTT can be used natively for other integrations to subscribe for notitications.
https://www.academia.edu/44759428/Secure_Push_notification_service_based_on_MQTT_Protocol_for_mobile_platforms

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Add mqtt notify service and update example page home-assistant.io#21344

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io

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

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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[probot-home-assistant]

Hey there @emontnemery, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[emontnemery]

Thanks, @jbouwh 👍