feat: separation - endpoints for finalize & assign

finalize at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}
assign at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}/blocks
I think the underly drivers should be responsible for throwing exceptions when any resource is not found, then we don't need validations/checks like that.

Let's leave this there.