Support nested `STRUCT` and `RANGE` types in `TableSchema` conversion

[haphungw]

Extend TableSchema to ProtoSchema conversion to support nested records STRUCT and interval ranges RANGE. Currently, these return SchemaError::UnsupportedType.

1. Recursively traverse nested TableFieldSchema.fields and build sub-message DescriptorProto definitions.
2. Generate hierarchical scope names (e.g. root__parent_col__child_col) matching Go managedwriter/adapt.
3. Cache and deduplicate identical child message schemas across different columns.
4. Generate synthetic start and end fields for RANGE corresponding to range_element_type (DATE, DATETIME, TIMESTAMP).
5. Narrow / remove SchemaError::UnsupportedType once we're done with nested types.