feat: Override Prefixes on Parent Structs

[c1moore]

We have been using envPrefix on our config struct, which is rather complex. It works great most of the time, but occasionally we need to pull in auto generated config that doesn't follow the structure defined in our config.

It would be nice to be able to override the prefixes and set the env var exactly as it's defined by the env tag. Perhaps this can be accomplished by adding a exact tag.