Support using rustls with AWS-LC crypto library

The arti, arti-client and tor-rtcompat crates now have rustls-ring and rustls-aws-lc features that enable using rustls with the specified crypto backend. The rustls feature is now an alias for rustls-ring for backwards-compatibility.

If both features are used, it defaults to using ring, which allows the features to be additive.

This doesn't address the licensing issue with ring because AWS-LC also contains code under the old OpenSSL license. But it might be advantageous for other reasons.

Fixes #1378.

requested review from @ahf

This is an attempt at implementing what was suggested in #1378 (comment 3046202); I think it mostly works.

I don't understand why the doc-features job is failing, and for the cbindgen one, do I just update the file?

mentioned in issue #1378

@nickm, can you opine on the cbindgen failure? It's surprising to me that it would fail as a result of the changes made.

@legoktm Sorry about the docsrs job failure, too. I think we can tolerate the failure, though - it's just a warning. So if the cbindgen job is fixed, the whole CI will be.

Please don't rebase/rewind the branch until/unless asked to do so by the reviewer (@ahf in this case). Thanks.

The cbindgen failure looks like a version mismatch; it should be fixed in main. Nothing to worry about.

Yep, I filed #1516 (closed) after figuring out it was unrelated to my MR.

Sorry about the docsrs job failure, too. I think we can tolerate the failure, though - it's just a warning.

Awesome.

requested review from @nickm and removed review request for @ahf