Normalize and validate field name

[haphungw]

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_<base64>) and attach the Storage Write API field extension (google.cloud.bigquery.storage.v1.column_name) = "<original_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.