Proper serversided config interpolation

[Veykril]

#13128 implements a very basic interpolation option for querying the manifest path of the workspace a command is being run for.

This gave me the idea that we could rename the overrideCommand configs to just command, remove the null default for them and redefine the current default command in terms of interpolations.
For that we need a proper interpolation system that allows us to do a variety of things, the aforementioned PR currently just allows $manifest_path to be substituted, but we want to be able to do more than simple substitution here (like simplistic conditional inclusion e.g ${var:value}, if var then value) ...

[Veykril]

After some thinking, this seems like a bad choice to do in a general thing as this is a huuuuge task. We are probably better off with just using a bit of simple interpolation bits for the overrideCommands that make integration with non cargo based build systems better