This commit makes the 3 existing InvocableHandlerMethod types more
consistent and comparable with each other.
1. Use of consistent method names and method order.
2. Consistent error formatting.
3. Explicit for loops for resolving argument values in webflux variant
because that makes it more readable, creates less garabage, and it's
the only way to bring consistency since the other two variants cannot
throw exceptions inside Optional lambdas (vs webflux variant which can
wrap it in a Mono).
4. Use package private HandlerMethodArgumentComposite in webflux
variant in order to pick up the resolver argument caching that the
other two variants have.
5. Polish tests.
6. Add missing tests for messaging variant.
7c36549e3a