Update to Pydantic 2.X

[NevilleS]

From fideslang created by ThomasLaPiana: IABTechLab#159

Is your feature request related to a specific problem?

There are some big performance improvements in Pydantic 2, that we'll want to take advantage of with our FastAPI server

Will need to be careful see and see how bad any potential breaks might be, although recently moving to 2.0 should make this a bit less painful

[emaheuxPEREN]

Is there any reason why restricting to pydantic<=2.7.1 and not broader pydantic<3?