Introduce content parameters.

[samuel-williams-shopify]

• add immutable, operation-specific parameter declarations with optional fields and constrained or open nested values
• support typed, untyped, and nested arrays with index-aware validation paths
• convert and filter parsed content while collecting path-aware validation errors
• require explicit upload declarations, stream accepted uploads through a callback, and discard undeclared or unhandled uploads
• add a dedicated Content Parameters guide