Knot.x Gateway API refactor

[tomaszmichalak]

Knot.x Gateway API requires significant architecture changes in the future (a streaming processing is not allowed now). Now we should consider smaller refactroing fixes:

• introduce a dedicated Proxy interface to mark all micro-services
• update a documentation and tutorials

Currently we use KnotProxy to code a Gateway module.

This approach has some benefits:

• we have a common approach for Knot Routing so we can reuse, for example, AuthorizationKnot in server/client-side integrations without changes in the code

Some disadvantages:

• it does not fix our Knot definition
• it omits all core features: Repository, Splitter, Assembler

[tomaszmichalak]

This is handled in https://github.com/Cognifide/knotx/issues/430. Closing the ticket.