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

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