repo: Add ostree_repo_write_regfile_inline

When working on ostree-ext and importing from tar, it's
quite inefficient and awkward for small files to end up creating
a whole `GInputStream` and `GFileInfo` and etc. for small files.

Plus the gtk-rs binding API to map from `impl Read` to Gio
https://docs.rs/gio/0.9.1/gio/struct.ReadInputStream.html
requires that the input stream is `Send` but the Rust `tar` API
isn't.

This is only 1/3 of the problem; we also need similar APIs
to directly create a symlink, and to stream large objects via
a push-based API.

src/libostree/ostree-repo.h

@@ -423,6 +423,18 @@ gboolean      ostree_repo_write_content (OstreeRepo       *self,
                                          GCancellable     *cancellable,
                                          GError          **error);
 
+_OSTREE_PUBLIC
+char *      ostree_repo_write_regfile_inline (OstreeRepo       *self,
+                                                const char       *expected_checksum,
+                                                guint32           uid,
+                                                guint32           gid,
+                                                guint32           mode,
+                                                GVariant         *xattrs,
+                                                const guint8*     buf,
+                                                gsize             len,
+                                                GCancellable     *cancellable,
+                                                GError          **error);
+
 _OSTREE_PUBLIC
 gboolean      ostree_repo_write_metadata_trusted (OstreeRepo        *self,
                                                   OstreeObjectType   objtype,