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.

apidoc/ostree-sections.txt

@@ -165,6 +165,12 @@ ostree_check_version
 ostree_commit_sizes_entry_get_type
 </SECTION>
 
+<SECTION>
+<FILE>ostree-content-writer</FILE>
+ostree_content_writer_get_type
+ostree_content_writer_finish
+</SECTION>
+
 <SECTION>
 <FILE>ostree-deployment</FILE>
 OstreeDeployment
@@ -355,6 +361,7 @@ ostree_repo_write_metadata
 ostree_repo_write_metadata_async
 ostree_repo_write_metadata_finish
 ostree_repo_write_content
+ostree_repo_write_regfile
 ostree_repo_write_regfile_inline
 ostree_repo_write_symlink
 ostree_repo_write_metadata_trusted