Remove importlib-metadata dependency (#5203)

* Remove importlib-metadata dependency

Remove the third-party importlib-metadata dependency in favor of the standard library's importlib.metadata, which is stable and available natively since Python >= 3.10.

Implements a unified, branchless caching wrapper in `opentelemetry-api` that:
- Bypasses standard `entry_points()` version-specific return type differences (SelectableGroups vs EntryPoints) by building a flat `EntryPoints` sequence directly from the low-level `distributions()` API.
- Caches all resolved entry points in-memory on the first call to avoid severe startup performance degradation caused by repeated filesystem scans in standard library `importlib.metadata` (resulting in a 37x speedup on subsequent lookups and a 2x faster cold start).
- Cleans up obsolete `importlib-metadata` and `zipp` requirements across 19 test and documentation configurations.

Assisted-by: Gemini

* Refactor importlib.metadata caching wrapper to address PR comments

- Leverage stdlib `entry_points()` directly instead of the custom `distributions()` loop, ensuring standard library's `sys.path` shadowing resolution (uniqueness filtering) is perfectly preserved.
- Map Python 3.10/3.11 `SelectableGroups` (dict-like object) to `EntryPoints` (sequence) using a compatibility helper `_as_entry_points`.
- Cache the result of the no-argument stdlib `entry_points()` call via `@cache` to eliminate redundant disk-scanning on repeated lookups.
- Add a compatibility unit test for `_as_entry_points` with dict-like input.

Assisted-by: Gemini

* Apply suggestion from @herin049

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>

* Add Sphinx RST module docstring to _importlib_metadata.py

Assisted-by: Gemini

* Undo rename of _original_entry_points_cached and document test behavior

- Revert the caching function name back to _original_entry_points_cached to
  avoid unnecessary churn.
- Add a docstring to test_as_entry_points_selectable_groups_compat
  explaining the normalization of Python 3.10 SelectableGroups vs 3.11+.

Assisted-by: Gemini

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Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>