Add service to retrieve schedule configuration

[rikroe]

Proposed change

This PR adds the service schedule.get_schedule to the schedule component, similar to calendar.get_events and weather.get_forecasts (#96370).

The main goal is to provide a way to retrieve schedule helper data through scripts/automations.
For example, I own bluetooth TRVs that have an internal schedule (high/low target temperature). I want to control this schedule via the HA UI and a schedule helper (on: high, off: low), instead of writing these schedules in YAML automations.

I am aware of #101906 and #80358 which both got rejected. However as far as I understand the objections by frenck and balloob, the main issue with both was imlementing a service that changes/configures a schedule, not only retrieves it.

If this PR is considered acceptable, I'll create a documentation PR accordingly.

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.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

To help with the load of incoming pull requests:

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

[home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (schedule) you are listed as a code owner for? Thanks!

Code owner commands

Code owners of schedule can trigger bot actions by commenting:

• @home-assistant close Closes the pull request.
• @home-assistant rename Awesome new title Renames the pull request.
• @home-assistant reopen Reopen the pull request.
• @home-assistant unassign schedule Removes the current integration label and assignees on the pull request, add the integration domain after the command.
• @home-assistant add-label needs-more-information Add a label (needs-more-information, problem in dependency, problem in custom component) to the pull request.
• @home-assistant remove-label needs-more-information Remove a label (needs-more-information, problem in dependency, problem in custom component) on the pull request.

[gjohansson-ST]

I think the better approach would be to open a discussion and close this PR in the meantime.

[rikroe]

Will open a discussion. Opened architecture discussion: home-assistant/architecture#1115.

Converted this PR to draft for now to have a sample implementation that can be referenced.