Expose public API for type string conversion and NamedStruct ↔ ExtensionColumn mapping

[wackywendell]

Problem

When implementing Explainable for extension relations that carry a base_schema (which is very common — most ExtensionLeafRel details include a NamedStruct), you need two things:

1. Convert between Substrait Type protos and the compact type strings used in the text format (e.g. i64, string?, timestamp_tz). substrait-explain already has this mapping internally in both the parser (parser/types.rs) and textify (textify/types.rs) modules, but it's not publicly accessible. Every Explainable implementation that needs to render a schema in output columns currently has to reimplement this mapping.

2. Convert between NamedStruct and Vec<ExtensionColumn>. A NamedStruct is a list of names paired with a struct of types — this maps directly to the => name:type, name:type output column syntax that ExtensionArgs already supports via ExtensionColumn::Named. But there's no utility to do this conversion, so every wrapper that carries a schema has to write the same loop pairing names with types and converting each type to/from a string.

Request

Expose public APIs for:

1. A function to convert a substrait::proto::Type to its text-format string representation, and the inverse parse function. Something like:

   pub fn type_to_string(ty: &Type) -> Result<String, ...>
   pub fn string_to_type(s: &str) -> Result<Type, ...>

2. Convenience functions to convert between NamedStruct and Vec<ExtensionColumn>:

   pub fn schema_to_columns(schema: &NamedStruct) -> Result<Vec<ExtensionColumn>, ...>
   pub fn columns_to_schema(columns: &[ExtensionColumn]) -> Result<NamedStruct, ...>

These would make it straightforward to implement Explainable for any extension that carries a schema, without duplicating substrait-explain's internal type mapping.