Make a public OperationMapper#createResolver overload to help with creating custom field resolvers

[kaqqao]

OperationMapper#createResolver is currently private, but also not exactly convenient to use, as it requires creating an Operation to represent the underlying Java class members. It would be useful to expose a more convenient overload that could be called by various end-user extensions (e.g. custom TypeMappers etc) to create custom resolvers.

The idea originally came from: #491