Fix several waiter path matcher issues

[jdisanti]

While implementing waiters in another branch, I discovered and fixed a number of issues with the path matcher codegen logic. These issues were:

• Generated code for nested flatten projections failed to compile due to the first projection producing a Vec<Vec<&T>> instead of Vec<&T>.
• Path matchers that don't use input were taking input as an argument anyway, which results in an unnecessary clone of the input when used by the generated waiter logic.
• The comparisons generated by RustWaiterMatcherGenerator would fail to compile whenever comparing a string against an enum.

This PR fixes all these issues.

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[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.