fix(gcm): reset AEAD packet/nonce counters during key exchanges

[vicajilau]

Fixes #165.

This PR resets the AES-GCM AEAD packet/nonce invocation counters _localAeadPacketCount and _remoteAeadPacketCount to 0 whenever new cipher keys are derived and applied (i.e. on NEWKEYS / key exchanges), matching the requirements for GCM in the SSH transport protocol.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.83%. Comparing base (0d121d2) to head (6e7d988).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   52.83%   52.83%   -0.01%     
==========================================
  Files          63       63              
  Lines        5125     5127       +2     
==========================================
+ Hits         2708     2709       +1     
- Misses       2417     2418       +1     

Flag	Coverage Δ
unittests	52.83% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
lib/src/ssh_transport.dart	51.37% <100.00%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

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