Add v6 SKESK

[TJ-91]

I added generating and parsing of v6 SKESK packets. They work similarly to the already implemented AEAD stuff, but use SEIPDv2 instead, and also, use HKDF for key derivation.

I added the Crypto Refresh test vectors for EAX/OCB, as well as an encrypt-decrypt test.

Further, I added the rnp_op_encrypt_enable_skesk_v6() API call that enables creating v6 SKESK. In the CLI it can be activated with --enable-v6-skesk.

The default behaviour for SKESK should be the same as before, even when compiling with ENABLE_CRYPTO_REFRESH.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.63%. Comparing base (a8f2318) to head (9ea7901).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2207      +/-   ##
==========================================
+ Coverage   83.28%   83.63%   +0.34%     
==========================================
  Files         107      107              
  Lines       22878    23168     +290     
==========================================
+ Hits        19055    19376     +321     
+ Misses       3823     3792      -31     

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[falko-strenzke]

When this gets merged RNP would also need a way to control whether to use the v6 SKESK or the v5 SKESK via FFI or CLI.
@TJ-91 @ni4

[falko-strenzke]

Question: what is the meaning of skeycount in this context?

[TJ-91]

    pkeycount = handler->ctx->recipients.size();
    skeycount = handler->ctx->passwords.size();

These are the number of SKESK or PKESK packets.

When writing the code I assumed that only one of those variables is non-zero, meaning, we have distinct cases for symmetric and asymmetric encryption. I now think that is a wrong assumption and you can actually use PKESK and SKESK packets simultaniously for the same SEIPD message.

Thus, I will need to adapt the logic here to also take into account the v2-SEIPD capability of the "PKESK recipients".