osbuild: add --output-directory=DIR

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.

docs/osbuild.1.rst

@@ -50,6 +50,7 @@ is not listed here, **osbuild** will deny startup and exit with an error.
 --checkpoint=CHECKPOINT         stage to commit to the object store during
                                 build (can be passed multiple times)
 --json                          output results in JSON format
+--output-directory=DIR          directory where result objects are stored
 
 MANIFEST
 ========