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

[jklingen]

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<String, Object>) 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.