feat(config): add additionalProperties support to generated config models (#5131)

* add additional_properties support to generated config models

Schema types with additionalProperties (Sampler, SpanExporter,
TextMapPropagator, ExperimentalResourceDetector, etc.) now capture
unknown keyword arguments in an additional_properties dict field.
This enables plugin/custom component names to flow through typed
dataclasses without modifying the codegen tool.

Implementation:
- _additional_properties_support decorator in _common.py wraps the
  dataclass __init__ to route unknown kwargs into additional_properties
- Custom dataclass.jinja2 template for datamodel-codegen conditionally
  applies the decorator and field when additionalPropertiesType is set
- pyproject.toml updated with custom-template-dir and additional-imports
- models.py regenerated via tox -e generate-config-from-jsonschema

Assisted-by: Claude Opus 4.6

* update CHANGELOG with PR number #5131

* fix CI: plain assignment, pylint suppression for intentional unknown kwargs

- Replace object.__setattr__ with plain self.additional_properties = extra
  (no frozen dataclasses, simpler and more Pythonic)
- Add pylint disable=unexpected-keyword-arg on test lines that intentionally
  pass unknown kwargs to verify the decorator captures them
- Add comment explaining setattr usage for __signature__ (pyright workaround)

Assisted-by: Claude Opus 4.6

* use ClassVar annotation instead of dataclass field for additional_properties

ClassVar tells type checkers the attribute exists without creating a
dataclass field. This means additional_properties doesn't appear in
__init__ signature, dataclasses.fields(), asdict(), or repr() — only
schema-defined fields show up. The decorator sets it as a plain
instance attribute at runtime.

Assisted-by: Claude Opus 4.6

* rename _additional_properties_support to _additional_properties

Shorter name, aligns with the attribute it creates.

Assisted-by: Claude Opus 4.6

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>