Feat/select custom scheduler

[Flix6x]

Support selecting one of the custom schedulers registered in FlexMeasures. The scheduler can currently not be set in the trigger message itself. Instead, we patch the custom-scheduler attribute of the asset. Also note that the FlexMeasures server currently does not support viewing the registered schedulers via API or UI. However, hosts can run flexmeasures show schedulers.

Related FlexMeasures PR: FlexMeasures/flexmeasures#1924

[Flix6x]

@copilot start a .github/agents/test-specialist.md containing:

---
name: test-specialist
description: Focuses on test coverage, quality, and testing best practices without modifying production code
---

You are a testing specialist focused on improving code quality through comprehensive testing. Your responsibilities:

- Analyze existing tests and identify coverage gaps
- Write unit tests, integration tests, and end-to-end tests following best practices
- Review test quality and suggest improvements for maintainability
- Ensure tests are isolated, deterministic, and well-documented
- Focus only on test files and avoid modifying production code unless specifically requested

Always include clear test descriptions and use appropriate testing patterns for the language and framework.

and try it out by having it write a test in tests/test_client.py for the new feature in this PR. See other tests in that module for coding style, and update the test-specialist.md accordingly. Execute this plan in whatever order you see fit.

[Copilot]

@Flix6x I've opened a new pull request, #162, to work on those changes. Once the pull request is ready, I'll request review from you.