Allow python middleware to set the URI of a request (#3002)

## Motivation and Context
Python middleware were unable to change the URI which makes it
impossible to change the route of an incoming request in Python.

## Description
URI has a #[setter] that Python side can use to change it.

## Testing
- Test has been added that ensures URI can be changed from python.
- If the URI is modified by a middleware in a local smithy-rs Python
server, a different route will be invoked.

## Checklist
- I've updated the changelog.next.toml

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>