Add a Nexus internal API endpoint for disk remove read only parent

[leftwo]

When we create a disk, the saga generates both a disk_id and a volume_id

        builder.append(Node::action(
            "disk_id",
            "GenerateDiskId",
            ACTION_GENERATE_ID.as_ref(),
        ));
       
        builder.append(Node::action(
            "volume_id",
            "GenerateVolumeId",
            ACTION_GENERATE_ID.as_ref(),
        ));

We use both ids when we create the disk database record:

    let disk = db::model::Disk::new(
        disk_id,
        params.project_id,
        volume_id,
        params.create_params.clone(),
        block_size,
        db::model::DiskRuntimeState::new(),
    )

However, the volume_id here is only used as the key to store the volume data
in the database. That volume_id is never exposed to anyone outside Nexus.

For the actual volume data structure and later the VolumeConstructionRequest,
we use the value in the disk_id, but call it volume_id

This is nice because it means the volume_id that propolis receives in the VCR
will match what the console has for a disk uuid, and it means stats will be
recorded under the same UUID.

However, Propolis (who needs to call Nexus and tell it to remove a read only parent
only has the top level disk UUID. The internal "volume_id" is not exposed.

This PR adds a new internal API endpoint that will take the disk UUID, then lookup
the volume_id and then call the volume layer remove read only parent saga. Adding
a layer like this also allows tests that operate only on a volume level to not require
any changes.