repo: Add ostree_repo_write_regfile

This API is push rather than pull, which makes it much more
suitable to use cases like parsing a tar file from external
code.

Now, we have a large mess in this area internally because
the original file writing code was pull based, but static
deltas hit the same problem of wanting a push API, so I added
this special `OstreeRepoBareContent` just for writing regular
files from a push API.

Eventually...I'd like to deprecate the pull based API,
and rework things so that for regular files the push API
is the default, and then `write_content_object()` would
be split up into archive/bare cases.

In this world the `ostree_repo_write_content()` API would
then need to hackily bridge pull to push and it'd be
less efficient.

Anyways for now due to this bifurcation, this API only
works on non-archive repositories, but that's fine for
now because that's what I want for the `ostree-ext-container`
bits.

src/libostree/ostree-repo-private.h

@@ -462,6 +462,16 @@ _ostree_repo_bare_content_commit (OstreeRepo                 *self,
                                   GCancellable               *cancellable,
                                   GError                    **error);
 
+OstreeContentWriter *
+_ostree_content_writer_new (OstreeRepo            *repo,
+                           const char            *checksum,
+                           guint                  uid,
+                           guint                  gid,
+                           guint                  mode,
+                           guint64                content_len,
+                           GVariant              *xattrs,
+                           GError               **error);
+
 gboolean
 _ostree_repo_load_file_bare (OstreeRepo         *self,
                              const char         *checksum,