Animation: Insert Keyframe only on active FCurve

The insert key operator allowed to insert a keyframe on all selected FCurves
and on the Active FCurve at cursor value.
But it didn't allow to insert a keyframe only on the active FCurve.

This patch adds that. It is available in the redo panel and under Key->Insert Keyframes (which can also be called with the I hotkey)

Pull Request: https://projects.blender.org/blender/blender/pulls/106307

source/blender/editors/space_graph/graph_edit.c

@@ -80,6 +80,11 @@ static const EnumPropertyItem prop_graphkeys_insertkey_types[] = {
      0,
      "Only Selected Channels",
      "Insert a keyframe on selected F-Curves using each curve's current value"},
+    {GRAPHKEYS_INSERTKEY_ACTIVE,
+     "ACTIVE",
+     0,
+     "Only Active F-Curve",
+     "Insert a keyframe on the active F-Curve using the curve's current value"},
     {GRAPHKEYS_INSERTKEY_ACTIVE | GRAPHKEYS_INSERTKEY_CURSOR,
      "CURSOR_ACTIVE",
      0,