[feature] an option for the file name to be only the last nested module

[jaskij]

The easiest path to have my Rust modules mirror Protobuf packages is to have a directory structure like foo/bar/v1.rs, especially if I want to other files in the same modules (say, foo::bar::random for a random generation module).

To that end, it would make a lot of sense for me to have an option (for both protoc-gen-prost and protoc-gen-prost-serde) to only use the last package name in the generate Rust file name (essentially the basename of the proto file). This would probably mean that protoc-gen-prost needs to create an appropriate directory tree too.