Python: Add new annotation_pre & annotation_post handlers

Annotation tool is used as a general mark tool for many add-ons. To be able to detect when an annotation is done is very handy to integrate the annotation tool in add-ons and other studio workflows.

The  new callback names are:  `annotation_pre`  and `annotation_post`

Both callbacks are exposed via the Python module `bpy.app.handlers`

Example use:

```
import bpy

def annotation_starts(gpd):
    print("Annotation starts")

def annotation_done(gpd):
    print("Annotation done")

bpy.app.handlers.annotation_pre.clear()
bpy.app.handlers.annotation_pre.append(annotation_starts)
bpy.app.handlers.annotation_post.clear()
bpy.app.handlers.annotation_post.append(annotation_done)
```

Note: The handlers are called for any annotation tool, including eraser.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D14221

source/blender/python/intern/bpy_app_handlers.c

@@ -64,6 +64,8 @@ static PyStructSequence_Field app_cb_info_fields[] = {
     {"load_factory_preferences_post", "on loading factory preferences (after)"},
     {"load_factory_startup_post", "on loading factory startup (after)"},
     {"xr_session_start_pre", "on starting an xr session (before)"},
+    {"annotation_pre", "on drawing an annotation (before)"},
+    {"annotation_post", "on drawing an annotation (after)"},
 
 /* sets the permanent tag */
 #define APP_CB_OTHER_FIELDS 1