Suggestion: add pydantic versioned schema migration + base DLC schemas

[C-Achard]

This is a reminder/centralized post regarding where to host DLC config/project schemas and related tools.

Several of our libraries in the software stack could benefit from having access to:

• DLC project layout/file structure schemas
• DLC config schemas (available in main DLC)
• Versioned schema migration code (available in main DLC)
• DLC keypoint/data spec schema(s)

and potentially other pieces of code.

Criteria for inclusion in DLCLibrary (requires discussion, this is an outline):

• Lighweight dependencies
  • No core DLC
• Useful to at least two DLC repos in a concrete way
  • e.g. repo-specific migration logic should be hosted at the lowest stage where needed only
  • Adding code "for later" should be strictly documented and motivated, and ideally have an associated WIP PR in involved repos
• Should have one central domain-correct behavior
  • Centralizing prevents incompatible interpretations across repositories
  • DLCLibrary should be a source of truth, not a copy
  • Existing implementations should migrate to importing it, adding here should have corresponding migration logic ready downstream ideally
  • Must not become a drawer of loosely useful helpers that may be repo specific/general-purpose code
  • In general adding code here should be heavily scrutinized, only the bare required minimum should be accepted
• Testing can also be done in this repo on a different level:
  • Public schemas and migration behavior can be versioned and tested
  • Breaking changes require an explicit release and migration path
  • Round-trip and compatibility tests
  • Unknown or newer schema versions must fail clearly if broken/problematic