feat(derive): add a 'prefix = ' attr for structs

[wfraser]

I took the code sample I wrote up in #3513 and rebased it onto the current master (at 4.0.0-rc.2). I'm currently using this feature in a large codebase and some folks on at least two other companies I've spoken to have found it useful as well.

Implementation notes: this works very similarly to rename_all and rename_all_env in that it takes effect in the Name::translate() method (and also in Name::raw to prevent collisions), and how it is propagated from parent struct down to child fields. It's nice that the entire implementation lives in clap_derive and needs no support from clap itself.

I've added a test which illustrates how it works. In real usage, the sub-structs would be defined in different modules or even in different crates; the whole point of this is to permit uniform re-use of a set of related options across several binaries in a large codebase.

[epage]

While draft PRs can be useful for demonstrating ideas, I don't think we are at the point in #3513 to be ready for implementation. There are still areas of discussion that need resolution.