Add support for subentries to config entries

[emontnemery]

Proposed change

Add support for subentries to config entries

This implements initial core support for home-assistant/architecture#1070

HTTP / WS API TBD, this PR currently implements:

• HTTP /api/config/config_entries/subentries/flow to start a new subentry flow
• HTTP /api/config/config_entries/subentries/flow/{flow_id} to abort, get status or progress an existing subentry flow
• WS config_entries/subentries/list to list subentries of a config entry
• WS config_entries/subentries/delete to delete a subentry of a config entry

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
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format 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.
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• Untested files have been added to .coveragerc.

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[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!

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[emontnemery]

Tests are failing because some diagnostic and config flow tests need to be updated

[bramkragten]

Frontend is approved: home-assistant/frontend#23160