Convert scalar `TableSchema` to `ProtoSchema` AST

[haphungw]

1. Map all 14 BigQuery scalar types (STRING, BYTES, INT64, DOUBLE, BOOL, TIMESTAMP, DATE, TIME, DATETIME, GEOGRAPHY, NUMERIC, BIGNUMERIC, INTERVAL, JSON) to proto2 types (FieldDescriptorProto::Type).
2. Map NULLABLE, REQUIRED, and REPEATED modes to proto2 labels (LABEL_OPTIONAL, LABEL_REQUIRED, LABEL_REPEATED). Map unset modes to LABEL_OPTIONAL.
3. Construct the root DescriptorProto with 1-based sequential field tags.
4. Implement entry pointTryFrom<&TableSchema> for ProtoSchema and TryFrom<TableSchema> for ProtoSchema for public conversion.
5. Expose SchemaError with typed variants for unsupported and invalid types.
6. Verify that the generated descriptors load correctly into prost_reflect::DescriptorPool and roundtrip dynamic messages.