Grease Pencil: Add Sample Color operator to Vertex Color Mode

This PR adds the `sample_color` operator to Grease Pencil Vertex Paint
Mode.

Pull Request: https://projects.blender.org/blender/blender/pulls/146951

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -4331,6 +4331,7 @@ def km_grease_pencil_vertex_paint(params):
          {"properties": [("data_path", "scene.tool_settings.use_gpencil_vertex_select_mask_segment")]}),
         # Flip primary and secondary color
         ("paint.brush_colors_flip", {"type": 'X', "value": 'PRESS'}, None),
+        ("paint.sample_color", {"type": 'X', "value": 'PRESS', "shift": True}, {"properties": [("merged", False)]}),
 
         # Edit Lines overlay
         ("wm.context_toggle", {"type": 'Q', "value": 'PRESS', "shift": True},