SpringBoot Consider reverting predicate support for OperationMethod

Reviewing #45389 I noticed that OptionalParameter isn't documented and is quite hidden compared to the check that exists in OperationMethodParameter. Trying to get rid of the predicate explained why this was done this way:

Architecture Violation [Priority: MEDIUM] - Rule 'slices matching '(**)' should be free of cycles' was violated (1 times):
Cycle detected: Slice org.springframework.boot.actuate.endpoint.annotation -> 
                Slice org.springframework.boot.actuate.endpoint.invoke.reflect -> 
                Slice org.springframework.boot.actuate.endpoint.annotation

I don't know if we can get rid of the cycle but the complexity of the code that results from avoiding it is worse than doing a lookup by a FQN, IMO.

I am going to push what I have and then we can discuss what we want to do to move forward.

Comment From: philwebb

I've opened #47136 to completely drop support for @Nullable, but I think we should keep the predicate. IMO, the org.springframework.boot.actuate.endpoint.invoke.reflect package should not know about org.springframework.boot.actuate.endpoint.annotation (event if it's with a Stringly typed lookup).

I have made OperationMethod abstract to make it a bit more obvious that we expect a subclass to exist.

I think it's probably fine that the annotation processor only effectively supports DiscoveredOperationMethod. I suspect in reality we won't have any other OperationMethod implementations.