Customize request routing outside Knots

[mariuszkubis]

Currently, Knot.X routes every request through Repository Connector and Fragment Splitter ... Fragment Assembler.
In some scenarios, we want to be able to skip this path and define another.
A new structure should be introduced (e.g. called FlowRoute or Atom) which will contain details on how to process the request. Server will decide which requests should be routed through which Atom.

In the current design, a default Atom would wrap the flow from RepositoryConnector to FragmentAssembler.
In the new solution, we will be able to define new Atoms with a custom flow.

Every Atom should define input and output contract.

[tomaszmichalak]

This task was implemented in: #299.