Feature Request: Expose matched URL pattern on Request object

Hello,

I'm building a REST API with Chalice and I'm using [OpenAPI 3](https://swagger.io/specification/) to document the API and validate incoming requests. In order to find the right validations to enforce, it's necessary to compare an incoming request against the specification document, and match the right URL pattern. Today, this is hard to do with Chalice because the [Request object](https://github.com/aws/chalice/blob/master/chalice/app.py#L380) doesn't expose the URL pattern of the route handling that request.

The Chalice app does have this information (because it needs to find the right decorated function to call) but it's hard to access from inside the function.

I'm imagining something like this:

```python
@app.route('/hello/{name}')
def hello(name):
return {'pattern': app.current_request.matched_url_pattern,
# For extra credit (I know path is in the context already):
'path': app.current_request.path}

# GET /hello/brendan would return:
# {
# 'pattern': '/hello/{name}',
# 'path': '/hello/brendan'
# }

```
(obviously in this silly trivial case I could just assign `'/hello/{name}'` to a variable and use it both places)

I happen to be using [this library](https://github.com/grktsh/python-oas) but the feature I'm asking for would be library agnostic.

Thanks for all your work on Chalice!

Start in chalice/app.py around the Request object referenced in the issue, then trace how the Chalice app selects the decorated function for a route. Expose the matched URL pattern through app.current_request so the example can return '/hello/{name}' for /hello/brendan; verify the behavior with the project's request and routing tests if available.

Written by the indexing model from the issue text.