feat(config): add shared load_entry_point utility for declarative config plugin loading (#5093)

* add shared load_entry_point util for declarative config plugin loading

Extracts a generic `load_entry_point(group, name)` helper into `_common`
so that resource detector, exporter, propagator, and sampler plugin loading
in declarative file config can all use the same entry point lookup pattern
rather than duplicating it.

Refactors `_propagator.py` to use the new util, removing its inline
entry point lookup.

Assisted-by: Claude Sonnet 4.6

* update CHANGELOG with PR number #5093

Assisted-by: Claude Sonnet 4.6

* add test clarifying instantiation errors are not wrapped

load_entry_point returns the class; calling it is the caller's
responsibility. This test documents that errors from instantiation
propagate as-is rather than being wrapped in ConfigurationError.

Assisted-by: Claude Opus 4.6