Enable Pydantic plugin for MyPy

[tlento]

We don't use the pydantic plugin for mypy, and it turns out this allows for a lot of shady stuff up to and including passing the wrong BaseModel subclass into stock Python dataclass and mypy pretending that's ok.

This issue is to enable the pydantic plugin. Note, however, that we were (inadvertently) doing some shady stuff that will need fixing, or at least a type annotation override. The most common issue is the explicit Any, which would be good to clean up, but we have a couple of uses of generics which seemed sound but are probably inappropriate in some subtle fashion, and those should probably be addressed as part of this effort.

[tlento]

This requires little to no knowledge of Metricflow internals, but could be a nice way to get exposure to some bits of the codebase.