Add method to copy graph operation to new graph or eager operation builder

There are a number of situations that I have ran into where I need to copy a graph operation to another graph operation, or an eager operation, sometimes substituting inputs. The most important is lifting things to init scopes in functions (it's graph -> eager). This depends on being able to access attributes and input lists (#302) and being able to set them (tensorflow/tensorflow#50816, not 100% necessary but makes things much easier). I plan on implementing it myself once those are both merged, this is a reminder and documentation if someone else ends up doing it.

Begin by reviewing the Java graph and eager operation APIs, then read the dependencies in issue #302 and tensorflow/tensorflow#50816. Done means defining and implementing a way to copy an operation between graphs or into an eager operation builder, including possible input substitution, with tests covering the supported cases.