[mlir] Support slicing for operands in results in Python bindings

Slicing, that is element access with `[being:end:step]` structure, is
a common Python idiom for sequence-like containers. It is also necessary
to support custom accessor for operations with variadic operands and
results (an operation an return a slice of its operands that correspond
to the given variadic group).

Add generic utility to support slicing in Python bindings and use it
for operation operands and results.

Depends On D90923

Reviewed By: stellaraccident, mehdi_amini

Differential Revision: https://reviews.llvm.org/D90936