Checkpoint format

[jlamypoirier]

✨ Description

Add a CheckpointFormat class hierarchy to make the properties of the various checkpoint formats more understandable, and accessible at config validation time.

Also rename Converter -> CheckpointHandler

There shouldn't be any user-facing change here, they will be for future PRs [edit: accidently set the default export format to state dict, that was meant for next PR]

Extracted from #28 which was getting too big.

🔍 Type of change

Select all that apply:

• [ ] 🐛 Bug fix (non-breaking change that addresses a specific issue)
• [ ] 🚀 New feature (non-breaking change that adds functionality)
• [x] ⚠️ Breaking change (a change that could affect existing functionality)
• [ ] 📈 Performance improvement/optimization (improves speed, memory usage, or efficiency)
• [x] 🛠️ Code refactor (non-functional changes that improve code readability, structure, etc.)
• [ ] 📦 Dependency bump (updates dependencies, including Dockerfile or package changes)
• [ ] 📝 Documentation change (updates documentation, including new content or typo fixes)
• [ ] 🔧 Infrastructure/Build change (affects build process, CI/CD, or dependencies)