[mlir][Vector] Add support for Value indices to vector.extract/insert

`vector.extract/insert` ops only support constant indices. This PR is
extending them so that arbitrary values can be used instead.

This work is part of the RFC: https://discourse.llvm.org/t/rfc-psa-remove-vector-extractelement-and-vector-insertelement-ops-in-favor-of-vector-extract-and-vector-insert-ops

Differential Revision: https://reviews.llvm.org/D155034