feat(APM_TRACING): Test new APM_TRACING_MULTICONFIG merging logic

[tylfin]

Motivation

This is still in development for Java and Python here:

• add support for org level APM_TRACING configs dd-trace-java#9360
• chore(rc): apm tracing multi config support dd-trace-py#14364

But we want to have system-tests upfront to ensure it's functioning correctly for Sept releases.

Refs: DEBUG-4399, RFC here.

Changes

• Introduced the setup_tracing_client_http_header_tags_apm_multiconfig setup method, which prepares three different RC configurations and corresponding requests to test configuration precedence.
• Added the test_tracing_client_http_header_tags_apm_multiconfig test method to validate that header tags are correctly set or ignored based on configuration priority in the resulting spans.
• Updated the get_rc_params utility method to accept service_name and env parameters, allowing more flexible RC configuration targeting for the new tests.

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• If PR title starts with [<language>], double-check that only <language> is impacted by the change
• No system-tests internal is modified. Otherwise, I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added (or removed)?
  • Get a review from R&P team