feat: use Pydantic V2 models when possible instead of defaulting to Pydantic V1 models

[adubovik]

Resolves #220

Currently, DIAL SDK produces the Pydantic v1 models even when Pydantic v2 is installed:

Pydantic installed	DIAL SDK models
v1	v1
v2	v1

The fix introduces PYDANTIC_V2 environment variable to control the version of Pydantic models used when Pydantic v2 is installed.

So when PYDANTIC_V2=True:

Pydantic installed	DIAL SDK models
v1	v1
v2	v2

By default, PYDANTIC_V2 is set to False to maintain backward compatibility.