v6: add S2K Argon2 and AEAD s2k usage

[TJ-91]

implements Argon2 for S2K (secret keys, SKESKs), as well as AEAD encryption for secret keys.

Does not add new API functionality, however, that might be desired to give the user fine-grained control.

[codecov]

Codecov Report

❌ Patch coverage is 71.87500% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.40%. Comparing base (0d5c2b9) to head (1b8207f).

Files with missing lines	Patch %	Lines
src/lib/crypto/s2k.cpp	18.75%	13 Missing ⚠️
src/librepgp/stream-packet.cpp	75.00%	4 Missing ⚠️
src/librepgp/stream-key.cpp	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2296      +/-   ##
==========================================
- Coverage   85.46%   85.40%   -0.06%     
==========================================
  Files         126      126              
  Lines       22713    22732      +19     
==========================================
+ Hits        19411    19415       +4     
- Misses       3302     3317      +15     

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