Cleanup UnpackInterfacesMessage functionality

[aaronc]

Summary

The recent custom Any support with InterfaceRegistry and UnpackInterfacesMessage could use a little tidying up:

• [x] #6147: describe usage of the RegisterInterface protoName param andaccepts_interface & implements_interface proto extensions in ADR 019
• [ ] UnpackInterfacesMessage is an ugly name, let's rename it to something better. Maybe PostUnmarshaler or some variation of Rehydrate?
• [ ] With the accepts_interface proto extension we can implement this using codegen. This is not high priority but would be nice in the future
• [ ] Implement static linting of accepts_interface, implements_interface together with protoName

/cc @alexanderbez @jgimeno

[sahith-narahari]

@aaronc what do you think on adding a check to ensure a single interface isn't registered multiple times

[github-actions[bot]]

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