Normalize and validate field name

BigQuery table column names allow characters that are invalid in protobuf field identifiers (e.g., hyphens `user-id`, spaces `first name`, numbers as first character `123col`, or non-ascii characters).

1. If a field name is not a valid proto identifier, generate a placeholder name (e.g., `col_`) and attach the Storage Write API field extension `(google.cloud.bigquery.storage.v1.column_name) = ""`.
2. Return `SchemaError::DuplicateFieldName` if multiple fields in the same message share a name.
3. Return `SchemaError::EmptySchema` if a `TableSchema` contains no fields.