Decide on a pydantic v3 BaseModel tagged union method and document it here

At the moment we have multiple ways of using pydantic and tagged unions:
- `scanspec` using pydantic dataclasses and a decorator to make a tagged union of subclasses, it requires positional args
- `FastCS` using plain dataclasses and a pydantic BaseModel creation function to do serialization and deserialization
- `pvi` uses an explicitly created union of BaseModels
- `ibek` uses automatically created Entity BaseModels from EntityModel instances
- `blueapi` does something with BaseModels too, but I can't remember what

With the advent of pydantic v3, now would be a good time to look at investigating a common approach for all these usecases. Standardising on BaseModel instances would probably give us the most tool support for things like documenting models in sphinx, FastAPI and GraphQL support, and downstream serialization

@GDYendell @gilesknap @DiamondJoseph @tpoliaw @abbiemery FYI

@hyperrealist I'm hoping you'll take this on at some point in the new year