[mlir] Expose operation attributes to Python bindings

Operations in a MLIR have a dictionary of attributes attached. Expose
those to Python bindings through a pseudo-container that can be indexed
either by attribute name, producing a PyAttribute, or by a contiguous
index for enumeration purposes, producing a PyNamedAttribute.

Depends On D90917

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D90919