stub: introduce StubFeatures, similar to LoaderFeatures

systemd-boot reports its features via the LoaderFeatures EFI variable.
Let's add something similar for stub features, given they have been
growing.

For starters only define four feature flags. One is a baseline feature
we pretty much always supported (see comment in code), two are features
added in one of the most recently released systemd version, and the
final one, is a feature we added a few commits ago.

This is useful for userspace to figure out what is supported and what
not.

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