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/libostree-devel.sym

@@ -26,6 +26,9 @@ LIBOSTREE_2021.2 {
 global:
   ostree_repo_write_regfile_inline;
   ostree_repo_write_symlink;
+  ostree_repo_write_regfile;
+  ostree_content_writer_get_type;
+  ostree_content_writer_finish;
 } LIBOSTREE_2021.1;
 
 /* Stub section for the stable release *after* this development one; don't