Express dependencies between fields via attributes (e.g. `requires = "other_field"`)

[LukasKalbertodt]

This can be done in clap for example. Might be nice to express sth like that with attributes and have it checked automatically. However, it would increase the scope of this library considerably and it's straight forward to add such a check in your code afterwards.

[LukasKalbertodt]

This can be done with struct validation now, but of course that's not as nice as expressing dependencies as attributes directly.