This PR includes theses changes:
- Handling "optional" parameters is not optional anymore. Handling an additional parameter for it makes templates more complicated, and IMO, it is not worth it (as discussed in issue, it is an "acceptable" breaking change).
- Optional parameters are handled everywhere: path params, query params, form params... Generated sample code compiles (both hyper and reqwest libraries).
As we discovered in #3463, there are various bits of the rust client generator that are currently untested. This PR adds tests for various generator-specific types, and also files.
Follows the example of the rust-server generator for making it easy to add new test specs. This should make it easier for future contributors to make sure that their contributions are tested
