Feature request: Support for extrapaths field

[mpriya501]

We set few additional paths to PYTHONPATH and run the black formatter command. For example, env PYTHONPATH="/pathA:/pathB:/black-binary-path" /usr/bin/python3.11 -m black <file_name>. Those multiple packages are in primary location. In my understanding, black formatter extensions take the dependency and black packages from user defined python interpreter set at the interpreter setting.

The functionality that I think is to have the "extrapaths" field in black formatter where it allows to point the packages that are in different location. Then extension can read the package paths from "extrapaths" and insert it into sys.path. In this way, those location package paths are referenced first, and we'll be able to run the above command without setting PYTHONPATH.