Use HA native OAuth2 flow for google assistant components

[awarecan]

Description:

Use HA native OAuth2 flow for google assistant components

Google Account Link settings:

Advantage
Simplify configuration
Leverage HA user system, different google user can map to different HA user.

Breaking Changes

Need change google account link flow, need re-link account
Need remove client_id, access_token, and agent_user_id from configuration.yaml

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#6357

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

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

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

[balloob]

Awesome!

I think that for client ID, we should add a new page https://www.home-assistant.io/google_assistant/ which can be used as client ID and that whitelists the redirect URI. We did a similar thing for iOS: https://github.com/home-assistant/home-assistant.io/blob/next/source/ios/index.markdown (look at raw version)

Pending docs this looks good to go!

[awarecan]

I don't think we need https://www.home-assistant.io/google_assistant/ redirect URI support. Yes it may looks little better for user experience, but it adds unnecessary dependency between Google Assistant feature and document website.