storage: metadata_provider: Ensure idempotency when creating provider
Currently, in production, we have duplicated entries in origin_metadata and
This is due to a bad primary key on metadata_provider. This commit ensures
uniqueness when adding new metadata_provider by adding a new unique primary key
on (provider_type, provider_url).
This is preparatory work to allow a smooth transition for D2988 (which will
need to be rebased).
Related to T2075