[rustls] simplify feature selection for rustls

[alexmaco]

I am opening this issue here, since has no option to open issues in the mesalock-linux/rustls repository.

Looking at the first commit on the rustls fork that adds feature flags (currently fed32702b8d7afaec1271ef8494188d4f8ae688b), I can see there are some areas that can be simplified, and the feature selection can be performed with less code and more clearly.

As an example, in impl SupportedGroups for NamedGroups, there can be a single supported method, and each item in the vector can be conditioned directly on the related feature, which would make it much easier to see exactly what NamedGroups are included for which feature.

I would like to open a PR with these simplifications, and I would like to ask how do you think this should be handled: would you apply commits on top of the mesalink branch of the rustls fork, or would you apply it directly op top of fed32702b8d7afaec1271ef8494188d4f8ae688b and rebase on top of that ?

[ymjing]

It's OK to apply your commits on top of the mesalink branch. Looking forward to see you PR.