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-content-writer.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2021 Red Hat, Inc.
+ *
+ * SPDX-License-Identifier: LGPL-2.0+
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+#define OSTREE_TYPE_CONTENT_WRITER (ostree_content_writer_get_type ())
+_OSTREE_PUBLIC G_DECLARE_FINAL_TYPE (OstreeContentWriter, ostree_content_writer, OSTREE, CONTENT_WRITER, GOutputStream)
+
+_OSTREE_PUBLIC
+char * ostree_content_writer_finish (OstreeContentWriter  *self,
+                                     GCancellable         *cancellable,
+                                     GError              **error);
+
+G_END_DECLS