[C++][Python] Add VariableShapeTensor implementation

[rok]

Describe the enhancement requested

With VariableShapeTensor definition discussed in #24868, #37166, mailing list and approved on the mailing list we now want to add it's C++ implementation and a Python wrapper.

Component(s)

C++, Python

[raulcd]

@rok I am planning to move this to 19.0.0 as I am going to do the feature freeze, is that ok?

[rok]

This one still needs reviews