Add FHIR versioning

[adamkells]

Description

This PR adds configuration for FHIR versioning (stu3, R4b and R5). It also includes a number of utilities for conversion between formats, checking version types and loading resources. The documentation and tests have also been updated accordingly.

Related Issues

Closes #120

Type of Change

• 🐛 Bug fix (non-breaking)
• ✨ New feature (non-breaking)
• 💥 Breaking change
• 📚 Documentation
• 🧪 Tests only
• 🔴 Core change (requires RFC)

Testing

Added comprehensive unit tests.

• uv run pytest passes locally and generates no additional warnings or errors.
• Added / updated tests to cover the changes.
• Manually tested (describe how)

Checklist

• I have read CONTRIBUTING.md and followed the guidelines.
• I have linked all relevant Issues / Discussions / RFCs.
• I have updated documentation where needed.
• I understand all code changes and can explain the design decisions and trade-offs.
• I am available to respond to review feedback.