feat: add a static copy of legacy proto-based types

[plamut]

Closes#999.

Adding unmaintained legacy types definitions.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[tseaver]

Nice -- I had been thinking to add a request for import-time warnings.

[plamut]

Thanks!

[plamut]

Oh, wait... didn't we say we want to add back the proto* dependencies, so that those imports will not result in an error? Maybe as a "legacy_types" extra?

And what about the intersphinx links? Should we perhaps expose these models in the docs with a big fat "OBSOLETE!" warning at the top?

@tswast

[tswast]

Oh, wait... didn't we say we want to add back the proto* dependencies, so that those imports will not result in an error? Maybe as a "legacy_types" extra?

Yes, please. I think we've added the BQ Storage API as a required dependency now, right? That should mean we pull those in transitively anyway, but it wouldn't hurt to be explicit.

And what about the intersphinx links? Should we perhaps expose these models in the docs with a big fat "OBSOLETE!" warning at the top?

Yeah, let's make sure we generate docs for it, but with a warning & link to our migration guide.