Change metadata_standard_ids cardinality to 1...n

[allysonlister]

Currently there is exactly 1 metadata standard id within metadata.

However, it would be fantastic if we could allow multiple ids (1...n instead). This would allow us to reference not only e.g. the FAIRsharing DOI of the standard (which a number of DMP tools will have) but also other ids, e.g. the homepage url or resource DOI of the standard, or other appropriate identifiers.

Please can I suggest a change such that 1...n metadata_standard_ids are allowed? Thanks!

(tagging @TomMiksa)