repo: Add an API to init `OstreeSePolicy` from commit directly

repo: Add an API to init `OstreeSePolicy` from commit directly

This is part of `OstreeCommitModifier`, but I'm not using
that in some of the ostree-ext Rust code.

It just makes more sense as a direct policy API, where it should
have been in the first place.  There's already support for
setting a policy object on a commit modifier, so that's all the
old API needs to do now.