BTN-1618: Unify the creation of PDM entities using Pydantic

https://www.youtube.com/watch?v=EJRL5iXg8Qo

This MR completes the first checklist item for BTN-1618. This MR includes...

• Converting all our handwritten models in /entities to be Pydantic models.
• Fixing up the tests to pass. Known breaking changes are
  • All constructors are keyword-argument only now.
  • Handwritten __repr__() has been removed almost everywhere.
  • We are now more consistent about always serializing enum values, not names (so "altaz" instead of "ALTAZ")
• Some minor fixes to the Marshmallow schemas (hope to perhaps delete these later on)

It's a lot of read & review but the majority of the changes are deleting boilerplate __init__(), __str__(), __eq__() functions and so forth.

My suggestion for reviewers would be to focus your time looking at src/ska_oso_pdm/entities/common/target.py and the test changes.