Python debugger extension support

[karthiknadig]

Currently we have two asks that can be potentially done as extensions:

1. Custom variable filtering. (ask from DS team): For these kinds of features, @int19h suggested providing hooks into the dap messaging itself, in before, after style.

2. Custom formatting for variables. (ask from DS team)

How we do debugger extensions is also open for discussion.

1. We could, use entrypoint feature in setuptools for this.
2. Providing a dir to pickup extensions from.

The idea is to eventually allow supporting new frameworks entirely via extensions.

[fabioz]

As a note, there's already an extension mechanism in pydevd for custom formatting of variables.

See: https://github.com/microsoft/debugpy/tree/main/src/debugpy/_vendored/pydevd/pydevd_plugins/extensions

[fabioz]

For an example which can provide a custom string representation see:
https://github.com/microsoft/debugpy/blob/main/src/debugpy/_vendored/pydevd/pydevd_plugins/extensions/types/pydevd_plugins_django_form_str.py

For an example which changes the actual structure see:
https://github.com/microsoft/debugpy/blob/main/src/debugpy/_vendored/pydevd/pydevd_plugins/extensions/types/pydevd_plugin_numpy_types.py

[int19h]

The original DS asks were resolved in other ways, and beyond that there are no specific scenarios for this. Closing until we decide we actually need it.