Add Initial Extension Support

[wackywendell]

Summary

Begins addressing #367 and #342 by adding support for parsing the types in YAML Simple Extension Files into Rustic types - with validation and validity enforced. This includes a string text parser, handling built-in types, compound types, named structs, custom types, and validated parameter constraints in the Simple Extension YAML files.

• Adds ExtensionFile to load and validate extension YAML and a reusable Registry for URI+name type lookup.

This is a draft PR - it's mostly ready, but I wanted to do a last couple checks before opening it up too much.

Scope

• Types-only: no functions or call validation yet.
• Public API exposes parsed types (ExtensionFile, Registry, CustomType, ConcreteType) and enforces validation of those on creation / read.

Key Changes

• Type system
  • New BuiltinType, CompoundType, ConcreteType, CustomType with Display/round‑trip support for alias and named‑struct structures.
  • Parameter constraints: data type, integer (with min/max), enum (validated/deduped), boolean, string.
  • Parsing to and from the YAML structures (TryFrom<TypeParamDefsItem>, Parse<RawType>)
• File/Registry
• Context and proto glue
  • Separates out ProtoContext from Context, to distinguish between things needed for Protobuf parsing (ProtoContext)
• Type expression parser
  • Parses simple, user‑defined (u!Name) and type variables; visits extension references for linkage bookkeeping.
• Build/CI
  • parse feature includes serde_yaml; include!(extensions.in) is gated behind extensions feature.
  • Aligns actions/checkout to v4, updates Cargo dependency set, and bumps the substrait submodule.

Compatibility Notes

• New trait bound ProtoContext on proto parsing that previously required only Context.
• extensions.in now compiled only with features=["extensions"].
• Minimal, types-only round‑trip implemented; other sections remain empty when converting back to text.

Testing

• New unit tests cover:
  • Type parsing and round‑trip for alias and named‑struct.
  • Parameter constraint handling including enum validation and integer bounds (with current truncation behavior).
  • Registry creation and type lookup; core registry smoke test behind features=["extensions"].