feat(config): generic resource detector loading in declarative config

[MikeGoldsmith]

Currently _run_detectors() in _resource.py has a hardcoded if branch for each supported detector type. As more detectors are added, this pattern becomes repetitive.

A follow-up to generalise the loading logic — e.g. iterating over detector_config fields and loading detectors by name (including out-of-source/contrib detectors) — would make the code easier to extend.

Raised in: #5004 (comment)

[ravitheja4531-cell]

Hi, I'd like to work on this!

My plan is to replace the hardcoded if/elif branches in _run_detectors() with a generic lookup using importlib.metadata.entry_points(group="opentelemetry_resource_detector") — the same mechanism already used in Resource.create() for OTEL_EXPERIMENTAL_RESOURCE_DETECTORS. This would also naturally support contrib/out-of-source detectors (AWS, Azure, GCP, etc.) referenced by name in declarative config without requiring any SDK core changes.

Before I open a PR, a couple of questions:

1. Should the fallback for unknown detector names raise an error or silently skip (similar to how OTEL_EXPERIMENTAL_RESOURCE_DETECTORS tolerates failures per Tolerate exceptions among experimental resource detectors #4373)?
2. Are there any built-in detectors (e.g., otel) that don't have an entry point and need special-casing?

Happy to adjust the approach based on any guidance. Thanks!