Python: Expose G-API Operations as Python types

[dmatveev]

Currently there is a way to define and implement own (custom) operations in Python, but there's no way to implement the existing G-API Operations in Python, as their type information is not exported.

Why have this functionality in place, as if a G-API operation already exist, it is implemented in C++? Examples:

• Use PIL/Pillow resize instead of the OpenCV resize to improve accuracy, if it is the case.
• ..more?

[dmatveev]

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