TST: add egg_generator for re-creating serialized json artifacts

[tangkong]

Description

• Adds a 🐔 so we can make 🥚 whenever our model schema changes

Motivation and Context

If we're re-doing how tagged unions etcs are defined, we'll need to re-generate the json serialized configs

How Has This Been Tested?

I ran python egg_generator.py once, and confirmed they re-made the configs

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I'm on board. Looks normal and useful.

If we find issues with this we can tweak it in the subsequent PRs that find the issue.

[ZLLentz]

@joshc-slac do you have a script you used to generate im2l0_test.json?
If you do, I'd like to add it to this (since my schema updates in #59 also break the im2l0 test case)

[tangkong]

I added the im2l0 egg generator manually. Josh knows this, but there's a few changes that need to be made before it's a sensible, operational test against the real IOC (don't caput to .RBV, handle enums, add tolerances for equality checks)