Let the user optimize the request matching

[ErikPelli]

Extends #527.
Currently the URL matching just iterate over the list of the available request handlers and execute the matching ones.
This could have performance issues if the user specifies 10k+ thousands handlers, checked for each request.

In the Request handler Store specified in the pull request, we should add a method where we pass the request and the user returns the matching handlers, handled in its implementation.

Add an example in the examples folder where we use a map matching with the target host, and the modification of available handlers directly from the requests store.

[ErikPelli]

This issue contain a breaking change and requires goproxy v2
Wait for #592