Add generics support for v2 (another attempt)

[dmontagu]

Note: this requires an updated version of pydantic_core, but is currently passing all tests against pydantic_core's main branch.

There is still one major failing test I haven't resolved yet, which is getting it to work properly with "deep" recursive generic models. Figuring that out is my goal for tomorrow, but I wanted to open this now for the purposes of review.

@samuelcolvin you mentioned that the dataclasses stuff might blow this up, I'll be happy to merge it all together when that's ready.

fix #5146

[samuelcolvin]

A few comments but on the whole I think we should merge this asap.

I still believe that generic_recursion_self_type and friends could be simplified if we adopt something like #5149. However:

1. I don't pretend to know enough to be sure
2. If those simplifications aren't possible, it's fine
3. this shouldn't block merging

[dmontagu]

I still believe that generic_recursion_self_type and friends could be simplified if we adopt something like #5149. However:

I agree it's hard to reason about, so I also don't pretend to be confident, but I think the main thing that has the potential to simplify the complex recursion stuff would be if the BaseModel's validator was a ModelValidator rather than TypedDictValidator, as we discussed. I think most of the weird have-to-go-deeper stuff comes from the fact that at the top level of the schema, you don't have a model_schema you can refer to. when building the inner typed_dict_schema.

At any rate, agree with all the above feedback, I'll make the changes and merge once they are complete.