Generate snake_case names in Rubyland (with backwards compat via generation + method_missing)

[mjgiarlo]

Why was this change made? 🤔

This commit allows models to keep JavaScript naming conventions in JSland (using camelCase for JSON-based Cocina data structures, e.g., in OpenAPI specifications) and Ruby naming conventions in Rubyland (using snake_case for Ruby bindings, e.g., hashes and method calls).

Fixes #335

Includes:

• Convert attributes from camelCase to snake_case at generate time
• Coerce attributes to camelCase when validating against the OpenAPI specification (i.e., using JSland conventions)
• Generate self.new method into models even if they are not validatable to allow us to coerce attributes to underscore form (for Rubyland joy) at runtime
• Add #method_missing to all affected models (via generation) to allow downstream codebases to continue using camelCase in Rubyland (backwards compatibility)
  • And leave tests untouched to demonstrate backwards compatibility
• Deprecate and warn that backwards compatibility goes away as of version 1.0.0
• Regenerate all models
• Regenerate Rubocop TODOs

How was this change tested? 🤨

CI

[mjgiarlo]

Superseded by https://github.com/sul-dlss/cocina-models/pull/375 (don't bother with metaprogramming, not needed).