Lib-kaitai-of-data-encoding: Extend support for the size header of fixed size string and bytes encodings

Size header of Data_encoding.string or Data_encoding.bytes (both fixed size) may have a various forms: ?length_kind:[`N | `Uint30 | `Uint16 | `Uint8] (see).

Currently, we hard-code the size header to Uint30. We want to extend this support.