Split allowed metadata store from the corpora module into a new Pinia store

The allowed metadata API endpoints have already been migrated, and are currently housed in api/metadata.ts: they must be relocated to a new api/allowedMetadata.ts file, along with their interfaces.

A new allowedMetadata store must be created, where the allowed metadata are stored by corpus ID { [corpusId]: AllowedMetadata[] }.

The setCorpusAllowedMetadata, updateCorpusAllowedMetadata and removeCorpusAllowedMetadata mutations can be removed altogether. The listAllowedMetadata, createAllowedMetadata, updateAllowedMetadata and deleteAllowedMetadata actions must be moved from the corpora store into that new allowedMetadata store.

This should be done before / to make #1140 (closed) easier, but does not require any other migration to be done, as allowed metadata are pretty standalone.