truly support pydantic v2 without shims

[technillogue]

to avoid vendoring or forking fastapi, we need to actually use the v2 pydantic models. this aims to detect which pydantic version is used and reimplement cog types for pydantic 2

[yorickvP]

The annotation is hopeless, typing.Annotated was added in python 3.9 and also doesn't seem to like being isinstance'd.

Tests in the commit before that are mainly failing because of openapi schema changes in 3.1.0: {'openapi': '3.1.0'} != {'openapi': '3.0.2'}, and also ge vs limit_value

[technillogue]

thanks! unfortunately CI only shows the tip of the iceberg. this PR should ideally figure out how to test this with both old and new pydantic, because those have completely different pytest errors. there is something completely incorrect about the v2 validators. some of the main errors that I don't know how to approach are:

FAILED python/tests/server/test_http_output.py::test_output_file[client0] - AttributeError: 'pydantic_core._pydantic_core.SerializationIterator' object has no attribute 'decode'
FAILED python/tests/server/test_http.py::test_train_openapi_specification[client0] - pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'no-info', 'function': <bound method Path.validate of <class 'cog.types.Path'>>})