Proposal: stateful stub server

[gko]

Today if we want to preserve the state after some request we need to implement this logic ourselves.
Example:

• I call POST on /users/add
• I want to get recently added user on /users
• I need to implement this storage myself

Instead we can make a simple link between /users/add and /users that will tell stub server that when I make a POST I want to make a simple storage that will return a new user in /users.

In case we need a special treatment we can add transform key as a function that will accept data and return the transformed data that will be stored.