Obtain variants for Rust apps (if applicable)

As it is done for C applications: https://github.com/LedgerHQ/ledgered/blob/master/src/ledgered/github.py#L73.

`VARIANTS` or `VARIANT_VALUES` is not official Makefile term, but a convention one.
What should we use for Rust/Cargo world ? Do we have already examples of Rust apps with different presumable variants ? 