deltas: Support including detached metadata in static deltas

deltas: Support including detached metadata in static deltas

This is very useful for the inline-parts case, as you can then include
detached signatures in a single file representing the commit.

It is not as important for the generic pull case, as the detached
metadata is only a single small file. Additionally the detached
metadata is not content referenced and may change after the static
delta file was created, so we need to pull the latest version anyway.