Add API for adding file contents directly to a package, without needing to create files

[dralley]

Currently we have API like this, for adding files:

    .with_file(
        "./test_assets/awesome.py",
        rpm::FileOptions::new("/usr/bin/awesome"),
    )?

and API like this, for adding scriptlets

    .pre_install_script("echo preinst")

the former assumes that you already have a file that exists, unlike the latter which takes the contents directly. This is a bit awkward.

We should probably consolidate this a bit, such that a user can easily create their package using either paths to existing files, OR directly providing the contents of the files.