Add origin info support for MQTT discovered items

[jbouwh]

Proposed change

Make it possible to define the source of MQTT discovery messages via a new optional dict origin. This will be logged when an MQTT entity or other component is first added and when the configuration is changed.

Examples

Assuming we want to discover a sensor entity and we publish a config payload to homeassistant/sensor/bla/config.
Example of a configuration payload without integration info:

{ "name": "Milk", "state_topic": "test-topic" } 

... would log: Found new component: sensor bla

With minimal integration info:

{ "name": "Beer", "state_topic": "test-topic", "o": {"name": "bla2mqtt" } }

... would log: Found new component: sensor bla from from external application bla2mqtt

Additional software version and a support URL can be supplied:

{ "name": "Milk", "state_topic": "test-topic", "o": {"name": "bla2mqtt", "sw": "1.1", "url": "https://bla2mqtt.example.com/support" } }

... would log: Found new component: sensor bla from external application bla2mqtt, version: 1.1, support URL: https://bla2mqtt.example.com/support

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)
• Deprecation (breaking change to happen in the future)
• 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 origin info for MQTT discovery home-assistant.io#28659

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
• I have followed the perfect PR recommendations
• 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.
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[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!

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[emontnemery]

This looks very good, some comments though

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[emontnemery]

Looks great, but we should adapt names in the code

[emontnemery]

Nice, thanks @jbouwh 👍