[mlir][tensor] Add gather/scatter op definitions to the tensor dialect.

Gather/Scatter are examined from first principles in light of our recent progress on tensor-based codegen
and in-place bufferization.

In the future, lowering of these abstractions to operate **inplace** on buffers
will likely require a more powerful buffer representation than strided memref.

General context: https://discourse.llvm.org/t/rfc-structured-codegen-beyond-rectangular-arrays/64707
Relevant TL;DR parts of the proposal:
- gather: https://discourse.llvm.org/t/rfc-structured-codegen-beyond-rectangular-arrays/64707#proposal-gatherop-and-friends-10
- need for more expressive types: https://discourse.llvm.org/t/rfc-structured-codegen-beyond-rectangular-arrays/64707#proposal-bufferization-copy-view-and-the-need-for-more-expressive-types-12
- jagged buffer discussion: https://discourse.llvm.org/t/rfc-structured-codegen-beyond-rectangular-arrays/64707#proposal-first-class-jagged-buffer-13

Differential Revision: https://reviews.llvm.org/D130348