Add support for integrations v2

[emontnemery]

Proposed change

Add support for integrations v2

Background in https://docs.google.com/document/d/1990CZGDDLnYPcCHVuspwwCvgOJsHXAmr-19gWWW635w

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [x] 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:

• [ ] 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.

The integration reached or maintains the following Integration Quality Scale:

• [ ] No score or internal
• [ ] 🥈 Silver
• [ ] 🥇 Gold
• [ ] 🏆 Platinum

To help with the load of incoming pull requests:

• [ ] I have reviewed two other open pull requests in this repository.

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[MartinHjelmare]

The PR is missing a description of the design of integrations v2.

[emontnemery]

The PR is missing a description of the design of integrations v2.

Right. Can we just add a link to the design document, I think that's public?

[MartinHjelmare]

We need dev docs and possibly a dev blog, and frontend. What else is left?

[robloh]

I'm sure this feedback is off topic here (sorry), but I stumbled across this exciting work and wanted to add some random thoughts after reading your design doc and didn't know where else to go with this:

1. In the doc Nest is given as an example under Google, but I would guess most consumers simply know it as Nest and wouldn't look under the Google brand?
2. BLE can have similar issues. For example: Cleargrass sensors show up twice in discovery, once as Cleargrass, and once as Xiaomi
3. The doc gives Legrand as having 2 options but there's actually 3 now. Legrand's latest offerings are now Zigbee (not so branded): https://developer.legrand.com/documentation/how-it-works/