refactor(config): rename 'known/unknown' to 'built-in/user-defined' terminology (#5214)

* rename 'known/unknown' to 'built-in/plugin' in declarative config

Update docstrings, comments, and test names to use clearer
terminology for components that are built into the SDK vs loaded
via entry points as plugins.

Covers the already-merged sampler PR and shared infrastructure.
The pending propagator, exporter, and resource detector PRs will
need the same rename applied when they merge.

Assisted-by: Claude Opus 4.6

* rename changelog fragment to PR #5214

* use 'user-defined' instead of 'plugin' per review feedback

Rename 'plugin' to 'user-defined' throughout — pairs better with
'built-in' and is less ambiguous.

Assisted-by: Claude Opus 4.6