Make validation pluggable by content type

[RobbeSneyders]

It would be great to make (de)serialization pluggable by content type type. The goal would be to be able to register a handler, a class or function with an interface we define, to (de)serialize request / response bodies for certain content types.

Currently Connexion only supports json and form content types, but by making this pluggable, people can implement handlers for unsupported content types, and possibly contribute them back to connexion.

Some previous work has been done on this in #760

Depends on #1525

[RobbeSneyders]

#1588 and #1591 tackle this for request and response validation respectively.