Add self-explaining method for adding selectors e.g. to HttpServerRequestActionBuilder

**User story**
As a citrus newbie, I would love to see a self-explaining method for adding selectors e.g. to `HttpServerRequestActionBuilder`, so that I know how to use selectors without reading the docs.

**Additional context**
Both `selector(Map)` and `selector(String)` are not really self-explaining (and JavaDoc does not help much, too). I suggest to overload this method with something like
`selector(CitrusHttpMethodHeader, String)`, where `CitrusHttpMethodHeader` would be an enum or typed constants.