metadata: add exported GstAnalyticsKeypointDescriptor API with GIR bindings

Move keypoint descriptor data (point names, skeleton connections) and
lookup logic from static inline helpers in the header into a proper
exported C source file (gstanalyticskeypointdescriptor.c).  The header
now declares a clean public API with GST_ANALYTICS_META_API-annotated
functions matching the style of gstanalyticskeypointmtd.h.

New public API:
  - gst_analytics_keypoint_descriptor_lookup()
  - gst_analytics_keypoint_descriptor_get_semantic_tag()
  - gst_analytics_keypoint_descriptor_get_point_count()
  - gst_analytics_keypoint_descriptor_get_point_name()
  - gst_analytics_keypoint_descriptor_get_skeleton_connection_count()
  - gst_analytics_keypoint_descriptor_get_skeleton_connection()

The functions are picked up by g-ir-scanner, so the DLStreamerMeta-1.0
GIR/typelib now includes KeypointDescriptor with full Python bindings
(DLStreamerMeta.KeypointDescriptor.lookup(), .get_point_name(), etc.).

All C++ consumers (yolo_v8, yolo_v26, keypoints_hrnet, keypoints_openpose,
blob_to_meta_converter, tensor.h) are updated to use the new pointer-based
lookup API instead of the removed static variables.

Also included:
  - Unit tests for the Python bindings (test_keypoint_descriptor.py)
  - Developer documentation (docs/user-guide/dev_guide/keypoint_descriptor.md)

Author: OskarFiedot

docs/user-guide/dev_guide/dev_guide_index.md

@@ -7,6 +7,11 @@
   - [Ubuntu advanced installation - build Docker image](./advanced_install/advanced_build_docker_image.md)
   - [Ubuntu advanced uninstall guide](./advanced_install/advanced_uninstall_guide.md)
 - [Metadata](./metadata.md)
+- [Keypoint Descriptor API](./keypoint_descriptor.md)
+  - [Built-in descriptors](./keypoint_descriptor.md#built-in-descriptors)
+  - [C API](./keypoint_descriptor.md#c-api)
+  - [Python API (GObject Introspection)](./keypoint_descriptor.md#python-api-gobject-introspection)
+  - [How descriptors relate to keypoint metadata](./keypoint_descriptor.md#how-descriptors-relate-to-keypoint-metadata)
 - [Model Preparation](./model_preparation.md)
   - [1. Model file format used by OpenVINO™ Toolkit](./model_preparation.md#1-model-file-format-used-by-openvino-toolkit)
   - [2. Model pre- and post-processing](./model_preparation.md#2-model-pre--and-post-processing)