Allow passing the env on Engine init

[pimeys]

Parameter env takes an object, that should reflect the system environment. E.g. process.env in node.

closes: prisma/prisma#7522
closes: prisma/prisma#7562

[tomhoule]

Do you think it would work to have a trait EnvProvider { fn var(&self, var_name: &str) -> Option<String>; }, then without duplicating, the existing methods could take &dyn EnvProvider. We would then have a NativeEnvProvider and a VirtualEnvProvider. Wdyt?

[pimeys]

Do you think it would work to have a trait EnvProvider { fn var(&self, var_name: &str) -> Option<String>; }, then without duplicating, the existing methods could take &dyn EnvProvider. We would then have a NativeEnvProvider and a VirtualEnvProvider. Wdyt?

Took simpler approach by just requiring Fn(&str) -> Option<String> instead of a full trait.