Support annotating methods with @Option and @Parameters

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[remkop]

Summary of implementing this in Groovy's upcoming picocli-based groovy.util.CliBuilder (#258):

Getter methods can be annotated with @Option:

• for interfaces (where the implementation is a picocli-backed java.util.Proxy)
• for methods that return a mutable data structure like a Collection or a Map

Here the IGetter binding gets the initial value from the getter method and keeps a reference to it. This reference is shared with the ISetter binding so the reference is replaced if picocli calls ArgSpec.setValue.

Setter methods must be annotated for implementation classes

• for single-value options or array values
• either getter or setter methods may be annotated for collection or map options

If picocli asks ArgSpec.getValue before the setter has been called, the initial value is either null or an empty collection or map. The parsed (or default) value is then set via the ISetter which invokes the setter method and keeps a reference to the value. This reference is shared with the IGetter so that the updated value is returned with ArgSpec.getValue is called after the setter has been called.