Add context to event trigger

[OnFreund]

Proposed change

As proposed in home-assistant/architecture#436, this adds backend support for event triggers to filter by context.

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)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml
trigger:
  - platform: event
    event_type: test_event_context
    event_data:
    context:
      user_id: my-user-id

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Add context to event trigger documentation home-assistant.io#14678

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

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• 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.
• Untested files have been added to .coveragerc.

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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with an integration (homeassistant) you are listed as a codeowner for? Thanks!
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[balloob]

Nice!

[balloob]

Just realized that matching straight up on context like this is going to be annoying if one wants to match multiple users.

[OnFreund]

Good point. How about something like:

if isinstance(value, list):
    vol.Required(key): vol.In(value)
else:
    vol.Required(key): value

when building the schema? This can also be used for event_data, not just context.

[OnFreund]

Downside is that this could be a breaking change if the user intended to match a list.

[OnFreund]

I changed it here:
#41036

To ensure backwards compatibility I only added this for context, not data, even though I think it could be a nice touch for data as well.

[MartinHjelmare]

The existing behavior was changed slightly with this check. The old check only set an event data schema if the config had a populated event data dict. The current check sets a schema as long as the event data key is in the config. It doesn't check for a truthy event data dict.

[OnFreund]

Good catch. I'm not sure this makes any practical difference (since the schema will be empty), but I'll fix this together with the multiple user selection below.

[OnFreund]

https://github.com/home-assistant/core/pull/41036/files#diff-aa600ac67a083c2d308272918338761fR36-R37