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.h

@@ -435,6 +435,16 @@ char *      ostree_repo_write_regfile_inline (OstreeRepo       *self,
                                                 GCancellable     *cancellable,
                                                 GError          **error);
 
+_OSTREE_PUBLIC
+OstreeContentWriter *    ostree_repo_write_regfile (OstreeRepo       *self,
+                                                    const char       *expected_checksum,
+                                                    guint32           uid,
+                                                    guint32           gid,
+                                                    guint32           mode,
+                                                    guint64           content_len,
+                                                    GVariant         *xattrs,
+                                                    GError          **error);
+             
 _OSTREE_PUBLIC
 char *      ostree_repo_write_symlink (OstreeRepo       *self,
                                        const char       *expected_checksum,