api: add method to get the iana value for the negotiated cipher suite

[camshaft]

Description of changes:

Currently there's no way to get the IANA value for a connection's negotiated cipher suite. There is the s2n_connection_get_cipher method which returns a const char *, but this is less than ideal when trying to match on values programmatically.

This change adds a s2n_connection_get_cipher_iana_value method which takes two byte pointers and sets those to the negotiated cipher suite. I feel like this is the interface with the fewest sharp edges.

The other options I considered were:

• Taking a uint16_t. It's nice to just pass a single pointer, but it leads to some ambiguity in the endianness of the contained value.
• Taking a uint8_t * and requiring that there be 2 elements. This is impossible to enforce and hard to document.

Testing:

I added some checks alongside the current tests for the s2n_connection_get_cipher method.

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 #2550 (d4292ea) into main (bcebb35) will increase coverage by 0.05%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main    #2550      +/-   ##
==========================================
+ Coverage   82.09%   82.14%   +0.05%     
==========================================
  Files         272      272              
  Lines       19188    19186       -2     
==========================================
+ Hits        15752    15760       +8     
+ Misses       3436     3426      -10     

[lrstewart]

Do you want to put this API with the "semi-public" QUIC APIs for now, or do you think it's worth adding to the main APIs?

[camshaft]

Do you want to put this API with the "semi-public" QUIC APIs for now, or do you think it's worth adding to the main APIs?

I think this is worth having outside of QUIC.