Add a new output-directory argument which specifies where to store
result objects. For now, this is purely optional and simply copies from
the old `output_id` into the specified directory. This allows a
backwards compatible transition towards removing any external access to
the osbuild cache.
Note that this has still lots of room for improvements:
* We only support assembler-output for now, but we could also easily
support entire trees as output, in case no assembler was selected.
Alternatively, we could introduce a "copy" assembler, that just
outputs the input tree.
* This parameter is optional, but should really be mandatory. There
is little reason to have the default behavior just dropping any
generated content. This would be a breaking change, though.
* We could move data out of a temporary object-store entry, rather
than copy it. But again, for backwards-compatibility, we leave the
latest store-object intact and do not move things out of it.
* We could now transition towards never committing anything to the
store, not even output IDs, unless explicitly checkpointed.
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