Allow OAI-PMH BaseURLs to have a `set` parameter for HAL

[paulwalk]

Benedicte Kuntziger has told us:

CCSD will soon modify the OAI access for all HAL portals.
Instead of providing a separate OAI endpoint for each portal, we will expose a single HAL endpoint and use an OAI set for each individual portal. This means that URLs will change from:

https://api.archives-ouvertes.fr/oai/inserm?verb=ListIdentifiers&metadataPrefix=oai_dc

to something like this :
https://oai.hal.science/?verb=ListIdentifiers&metadataPrefix=oai_dc&set=portal:inserm

This is a reasonable approach but not one we support in IRD which just records a standard BaseURL