Load default values from Rust expressions like constants

[LukasKalbertodt]

Right now default only allows a handful of different expression types. But sometimes you might want to store the value in a Rust constant or call a function or something like that. Maybe we want to allow that?

The obvious problem here is printing that in the config template. Either the user specifies default_in_template = ... for us to use, or we require that the value implements Serialize and somehow do it like this? Both seems rather meh.

And maybe basically no one wants this feature anyway. Or maybe we shouldn't even allow it as the default value should be expressible in simple expressions: that's whats in the config file in the end, after all.