feat: set certificate_authorities from trust store

[lrstewart]

Resolved issues:

Last chunk from #4502

Description of changes:

We need to parse every certificate in the trust store to collect a list of "Distinguished Names" (the "subject", like "C = US, ST = WA, L = Seattle, O = Amazon, OU = s2n, CN = s2nTestCert").

Call-outs:

• I couldn't find a way to do this with openssl-1.0.2. If we think that's a blocker, I can go read through more very old documentation :(
• I'm restricting the feature to client auth with custom trust stores with <10 CAs. I'm trying to avoid a misconfiguration leading to a very large extension, or an extension that "leaks" unintended information about the server's local environment.
• I made the new API return "count". I'm not sure about this choice: it seems to improve usability to me, but it's also not strictly necessary.

Testing:

• New unit tests
• I generated the extension on Openssl and compare it byte-to-byte to what we generate
• I tested that the extension causes Chrome to filter out certificates when prompting for a client auth certificate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.