Server-side planning models

[rambleraptor]

Closes #2302

Rationale for this change

This adds the models for server-side planning. There's a lot of model-related code (the requests + responses are large!), so I'm just adding the models now. I'll add the endpoints in a separate PR.

Are these changes tested?

No tests needed, just new models.

Are there any user-facing changes?

Nothing yet, the endpoints will be in a separate PR.

[Fokko]

Thanks for adding these @rambleraptor. I think it would be nice to:

• See if we can combine already existing classes like Transform to avoid duplication.
• Add some tests to see if we can (de)serialize the payload.

[rambleraptor]

@Fokko tests added! The tests caught a handful of Pydantic issues (thought they would've been caught at runtime...), so there's a couple changes sprinkled in the models.