Feature request - request side transformer

[ddl-ebrown]

As a follow-on to #289, we're seeing a need to have request-aware middleware plugged into the request processing pipeline to implement a pattern that cross-cuts all API endpoints.

The main idea is:

• Request structure matches some specific criteria (payload contains a field for instance)
• Request has additional context added to it

Right now we're doing this by wrapping the API handler functions that get setup at registration time -- but it would be better to have a well defined construct for this pattern.

Thanks in advance for considering!