Add support for subscribing to config entry changes

[bdraco]

Frontend: home-assistant/frontend#13585

Proposed change

Add support for subscribing to config entry changes.

The dispatcher is used to get events to the api since config entry state changes
are frequent and we do not want to broadcast them to the event bus.

I didn't use a collection here since config entries are rather noisy with retries
and state changes and I wanted to send everything in a single api so
there wasn't a race between fetching the initial payload and getting callbacks.

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:

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:

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  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.
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[probot-home-assistant]

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

[bdraco]

Will write tests after first round of feedback

[MartinHjelmare]

What will we use it for?

[bdraco]

Add support for subscribing to config entry changes home-assistant/frontend#13585

home-assistant/frontend#13446 (comment)

[balloob]

Looks good 👍