[encoder/ts-registry] Fix submit_ele_transfer_syntax! and extend Codec

[Enet4]

It turned out that submit_ele_transfer_syntax! was not usable unless to specify a stub. This is not ideal, but since type parameter defaults are not supported, the Codec expression in the macro has to be annotated with the remaining types. I added some associated constructor functions to help mitigate this.

Summary

• add utility constructor functions to Codec
• generalize submit_ele_transfer_syntax! macro so that it is usable in non-stub scenarios
• add TransferSyntax::pixel_data_reader and TransferSyntax::pixel_data_writer to inspect the pixel data decoder and encoder