Bump Pydantic to version 2

[NevilleS]

Migrated from IABTechLab#160

Closes #8

Description Of Changes

Pydantic 2 sees major changes that bring large performance advantages. Worth upgrading to stay up-to-date as well as enjoy downstream effects in increased performance

Pydantic 1 -> 2 Migration Guide: link

I fully expected we might need to tweak more things here based on testing against Fides directly, but we'll need to do more checks there to verify. Here is the corresponding Fides PR

As part of this migration, I adopted a very different general pattern for validation than we used before. Due to the major changes, I found it easier to use model_validator (formerly known as root_validator) and composed multiple validators together into a single, full-model validator instead of always using discrete validators by field.

Code Changes

• make updates required for Pydantic v2
• update the requirement bounds in requirements.txt
• remove Python 3.8 from supported versions

Steps to Confirm

• CI checks pass

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

[NevilleS]

Migrated this over from the original PR here: IABTechLab#160! Check that out for additional history 👍

[pattisdr]

Closing in favor of #11. I needed to understand the V2 changes independently so I started a new branch and did a first pass, and now I'm comparing our branches and seeing what pieces I've missed. This work has been very helpful as well as code comments on IABTechLab#160. Thanks Thomas 🙏