Add some quality-of-life gdbus helpers

[swsnr]

Add a helper to return a standard result type from an invocation, automatically wrapping the return value into a tuple if it isn't a tuple yet.

Add another helper to return an async result from a future.

Update the gdbus example to demo the new async helper, and show how to simplify parameter handling with auto-derived variant structs.

[swsnr]

Accidentally committed some left over change, I'll fix this tomorrow.

[swsnr]

@sdroege Pipeline passes now, but the question about the name still stands... should I rename the method?

[swsnr]

Thank you for reviewing and merging 🙏