Allow using pydantic for from_dict, schema for FastList

[NeejWeej]

Allow utilizing pydantic validaiton in from_dict via a new boolean flag. Add a core schema for the FastList to work with pydantic.

Running some simple benchmarks, it seems that the default to_dict and to_json for csp Structs are significantly faster than going the pydantic route. While pydantic does allow for custom serialization and deserialization, the callbacks in to_dict and to_json allow that flexibility as well. Thus, for most use-cases, I believe those are probably preferred.

However, going through pydantic for from_dict is faster and more correct than the current csp implementation (more correct as in respects the type annotations more strictly). Thus, it should be preferred. However, this might cause breaking changes for users, so I propose we introduce it as an optional flag in the from_dict function, and then later down the road switch the default to be True to use pydantic validation for constructing csp Structs from dicts. Here are some plots noting the benchmark results

[NeejWeej]

Adding a from_json that goes the pydantic route would also be pretty trivial, though maybe we want to put that in a separate PR

[timkpaine]

would be good to include in #332