Conformance checks during mux startup

[kele]

Create the ability of adding conformance checks that run after the mux has been set up. These would be able to:

• list all the plugins registered for each handler, together with the method, path and all other information
• ask plugins for their configuration

Conformance checks would need to cooperate with plugins, i.e. there is going to be no complex protocol for information exchange between a plugin and the conformance check. A simple func (p Plugin) Config() interface{} would be enough, as the conformance check needs to have full trust in the plugins anyway.