Consider vendoring pydantic into the pipeline repo

[evansd]

Pydantic v1 is no longer supported, and Pydantic v2 has compiled dependencies. This creates problems already described elsewhere:
opensafely-core/job-server#4557

And in this Slack thread:
https://bennettoxford.slack.com/archives/C069YDR4NCA/p1725547669768179

For a core, and relatively small, library like pipeline Pydantic is too heavyweight a dependency; so we should consider pure Python alternatives (whether third-party or home-rolled) to handle the relatively small amount of validation which pipeline involves.

In the short-term, the quickest way to remove the explicit dependency would be to vendor the pure Python version of Pydantic v1 into this repo.

We already have experience vendoring Python libraries in the opensafely-cli project so we can borrow workflows from there:
https://github.com/opensafely-core/opensafely-cli/blob/e9f7a44c1ea1ad55525e0d4c741447c2bb247eb7/DEVELOPERS.md#vendoring

[evansd]

Obviated by:

• Remove pydantic dependency #337