libostree: Add API to write metadata as stream

And use it in pull-local.  The goal here is to kill users of
ostree_repo_load_variant(), and move a bit more towards hiding the
variants.

doc/ostree-sections.txt

@@ -76,6 +76,7 @@ ostree_repo_write_metadata
 ostree_repo_write_metadata_async
 ostree_repo_write_metadata_finish
 ostree_repo_write_metadata_trusted
+ostree_repo_write_metadata_stream_trusted
 ostree_repo_write_content
 ostree_repo_write_content_trusted
 ostree_repo_write_content_async