refactor(config): rename 'known/unknown' to 'built-in/plugin' in declarative config

## Description The plugin loading code introduced in #5095, #5098, #5128, #5129, and #5131 uses "known" and "unknown" to describe components that are built into the SDK vs loaded via entry points. While technically accurate (mapping to JSON Schema `properties` vs `additionalProperties`), "built-in" and "plugin" are clearer from a user's perspective. ## Scope Rename in docstrings, comments, and PR descriptions across: - `_tracer_provider.py` — sampler and exporter factory functions - `_propagator.py` — propagator factory function - `_logger_provider.py` — log exporter factory function - `_meter_provider.py` — metric exporter factory function - `_resource.py` — resource detector factory function - `_common.py` — `_additional_properties` decorator docstring - `codegen/README.md` and `codegen/dataclass.jinja2` — template comments No functional changes — terminology only. Suggested by @tammy-baylis-swi in #5128.