Python: Out-of-box PIP experience

[dmatveev]

G-API module contains a lot of functionality with external dependencies, what makes it non-trivial to get a proper functioning version in Python.

Normally it assumes building the own version of OpenCV with all proper flags set, package it in Python, and load in Python via PYTHONPATH.

This is not the best Python experience, as PIP package for OpenCV is very popular, it exceeds 2M downloads per WEEK: https://pepy.tech/project/opencv-python

Here's the /incomplete/ list of capabilities which require build flags:

• GStreamer source
• oneVPL source
• OpenVINO backend
• ONNX RT backend

The following options are seen as the solution:

• Dynamic loading (aka plugins) for heavy dependencies looks as one solution to the problem.
• Another solution could be seen as re-integrating (injecting) the components into G-API at the Python level.