[mlir] improve and test TransformState::Extension

Add the mechanism for TransformState extensions to update the mapping between
Transform IR values and Payload IR operations held by the state. The mechanism
is intentionally restrictive, similarly to how results of the transform op are
handled.

Introduce test ops that exercise a simple extension that maintains information
across the application of multiple transform ops.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D124778