Provide configuration annotations

[lukasz-kaminski]

There should be a way to easily say what should be sent to OPA in a request.
The idea is to have annotations that could look like this:

@OpaRequest(document = "http/authz")
public class MyOpaInput {
    @HttpMethod
    private String myMethodFieldName;
    @HttpHeader("Authorization")
    private String whatev;
}

Applying it to a class would enable OpaFilter and would make it use this class as a template to send stuff to OPA.