Provide a base class that can be in a referenced or imported mode

[ivanimanishi]

Summary

Provide an official way to create custom node definitions that can dynamically switch between a "referenced" and "imported" modes.

User story

We currently have custom nodes derived from Gaffer.Box that switch from "referenced" and "imported" states dynamically.
Being in a "referenced" state mostly means it doesn't serialise its contents, instead reading an external file (like the Gaffer.Reference). That also necessarily means that it's in a read-only mode (because the contents are not going to be serialised).
In a "imported" state, it will serialise its contents, like a Gaffer.Box. And they may or may not be in a read-only mode.

This has mostly worked so far, but there is a a constant risk of things breaking because Gaffer will often only consider the Gaffer.Reference when it needs to support something that reads its contents from an external file (ex: https://github.com/GafferHQ/gaffer/blob/main/src/GafferBindings/MetadataBinding.cpp#L81).

It would be great if we could have an official way to create such types of node that would be properly supported by Gaffer.