Custom Request class

Hello,

I seem to always extend the base request class in order to add helper functions like file management, body arguments, query arguments, and others.

It would be great to have an extra param on Chalice's constructor to be able to pass a custom Request class for Rest APIs.

Here is a branch with the base change just to get the idea rolling:

https://github.com/fernandoflorez/chalice/tree/custom-request

Is this a good idea? Any thoughts?

Review Chalice's constructor and REST API request handling, then compare the linked custom-request branch with the current behavior. Done requires an agreed design for passing a custom Request class, including its scope and compatibility expectations.

Written by the indexing model from the issue text.