Add service entity context

[balloob]

Breaking change

Proposed change

This is a proposal.

I was talking with @marcelveldt about a specific media browser use case he has with Music Assistant. His integration maintains a queue per media player, and if a song is picked in the media browser from his integration's media source he wants to add it to the queue of the entity for which it was picked.

Media sources do not have any context about who is resolving media. This PR will change that by adding a context var with the current entity ID to the function that calls entity services.

Any function called as a result from an entity service can now detect the entity.

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

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.

[MartinHjelmare]

Should we wrap the current_entity.get call in a helper like we do for async_get_current_platform?

[balloob]

I did that. However I made it so it doesn't raise because methods that are using it can be re-used outside of entity service calls; like media browsing can be run from a WS too (when resolving for the browser)

[marcelveldt]

Just tested this branch with Music Assistant and it works perfectly fine. No more need for ugly workarounds to determine which player requested the media. Much appreciated!