feat(oai): add seda2.1rng export in oai

This allows the metadataPrefix seda2.1rng in oai such as :

/oai?metadataPrefix=seda2.1rng&verb=ListRecords

This uses the profile_generation defined here, that may need adaptations (see seda#15 (closed)).

This MR needs the cube seda with seda2.1 : seda!12 (closed)

linked to seda#22 (closed)