Add a public API endpoint to get the metadata of an origin

The indexer storage API provides an endpoint to get the metadata (origin_intrinsic_metadata_get).

It would be useful to have this endpoint public, similarly to origin_intrinsic_metadata_search_fulltext.

This requires changing code only in swh-web; that is very similar to what is already done for the fulltext search, except using origin ids as arguments instead of text.

Migrated from T1613 (view on Phabricator)

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