repo: Add `query_file` API

repo: Add `query_file` API

The underlying `ostree_repo_load_file()` API has the caller pass
`NULL` for output arguments it doesn't want.  This isn't sanely
bindable in Rust - what the generator does is always request
all values, but maps them all to `Option<T>`.

The main cases are where a user wants either metadata, or both
metadata and content.  This API gives just metadata; it's a
bit more efficient as we don't need to open the file, and doesn't
require the caller to `unwrap()`.