fix: rustls hard-dependency

[benpueschel]

Remove hard-dependency on aws-lc-sys by adding two new features: rustls-ring and rustls-aws-lc-rs.

By default, rustls-aws-lc-rs is enabled (which is also a default feature for hyper-rustls), but the dependency on aws-lc-rs can be overriden by consumers of the library.

This should fix compilation on ARM, but we should maybe add a section about this in the README to let consumers know how to fix compilation issues for ARM.

Let me know if I should add a section to the README, or if there's better place to let user's know about this fix.

Closes: #706

[XAMPPRocky]

Thank you for your PR! However shouldn't the default feature be the one that allows it to compile on ARM?

[benpueschel]

Thank you for your PR! However shouldn't the default feature be the one that allows it to compile on ARM?

Yeah, you're right. I'll make ring the default.

[benpueschel]

Looks like ring somehow breaks Windows builds.

[theRookieCoder]

I think I should make it clear that aws-lc-sys does claim to support aarch64 builds. But, it does not cross compile to arm from an x86 host like GitHub Actions. I'll try to compile natively on Android to check if arm builds work as expected

[theRookieCoder]

It seems aws-lc does not support the Android triple (aarch64-linux-android) at all. I was not able to compile it on Android either, but I do know ring supports it natively.

Looks like ring somehow breaks Windows builds.

I'm not sure how that happened, but most dependencies (at least the ones I depend on) use ring by default when using rustls, I suppose because it was (de facto) standard for a while.

[cpu]

It seems aws-lc does not support the Android triple (aarch64-linux-android) at all.

That target triple is described as both supported and tested in the aws-lc-rs platform docs. You might consider opening an upstream issue if you're having trouble. In my experience they're very helpful for these sorts of problems.

[benpueschel]

@XAMPPRocky do you think we could re-run the checks? I think some dependency briefly broke Windows builds, but that should be resolved now (hopefully 😄).

And I'd say as long as we don't know if aws-lc-rs supports ARM cross-compilation, going with ring by default should be fine.