Create KEYS_BUNDLE_ACCESS for sequester services

[mmmarcos]

In Parsec V3 we no longer need to use keys in the file manifest for each block, since we can use the workspace encryption key, EXCEPT for sequester service (since the workspace encryption key is not shared with sequester services, but instead manifests are re-encrypted with keys supplied by these services).

A simple solution would be to stop storing a key in the file manifest for each block, but that's a shame compared to the possibility of just creating a KEYS_BUNDLE_ACCESS for each sequester service (with this, users don't have to upload the metadata several times, so the implementation is just simpler that way \o/)