Implementation of custom annotation tool

Hello. There exists bokeh.models.BoxEditTool to draw rectangles which is nice. However i need to add additional functionality, so I was thinking about implementing my own rectangle annotation tool.

Is this possible?
Can it be done as a sort of plugin or do i have to fork and recompile full bokeh source?
How do i get started with this?

Thanks in advance

There is a chapter on creating custom extensions in the users guide:

Extending Bokeh — Bokeh 2.4.2 Documentation