Implement Jmespath shape traversal codegen for waiters (#3526)

This PR adds a `RustJmespathShapeTraversalGenerator` class that will
generate code to traverse Smithy generated shapes based on a Jmespath
expression. This is a necessary prerequisite for implementing Smithy
waiters support.

The generator only implements a subset of the full Jmespath spec. This
subset was determined by examining all the waiter expressions in the
existing SDK models.

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