api: add method to append protocol preferences

[camshaft]

Description of changes:

The current s2n_{config,connection}_set_protocol_preferences use null-terminated strings to pass ALPN values. This is a bad idea for a few reasons:

• right now it's impossible to have an ALPN value with a \0 (which is technically allowed by the spec).
• Passing a value with a null byte in the middle will silently accept half of the provided string without any indication that the rest was rejected.
• It forces the caller to copy strings into a temporary buffer when the string isn't null terminated.
• it's inefficient to iterate through the whole string just to find the length.

This PR adds s2n_{config,connection}_append_protocol_preference methods which take an explicit pointer and length and then copy the value into the protocol_preferences blob. This should fix all of the issues stated above.

I've also added a minimum length check (required by the spec), added tests, and cleaned up a memory leak that happened in the previous version when a protocol value failed to validate.

I used doxygen-style documentation for the new methods in anticipation of that being used in the near future.

Testing:

There is now a dedicated s2n_protocol_preference_test unit test for checking the behavior of the API. Before it was implicitly checked in other tests. I've added a lot of validation checks in this new module as well.

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

[codecov]

Codecov Report

Merging #2534 (9c16020) into main (5962569) will increase coverage by 0.07%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##             main    #2534      +/-   ##
==========================================
+ Coverage   82.10%   82.17%   +0.07%     
==========================================
  Files         272      272              
  Lines       19095    19095              
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+ Hits        15677    15691      +14     
+ Misses       3418     3404      -14