Add support for AES GCM.

[dajiaji]

In addition, make key parameter optional for KeyBuilder.from_symmetric_key.

[codecov]

Codecov Report

Merging #41 (7fe2eb9) into main (61c12d4) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   98.23%   98.53%   +0.30%     
==========================================
  Files          14       14              
  Lines         905      958      +53     
  Branches      210      229      +19     
==========================================
+ Hits          889      944      +55     
+ Misses         13       11       -2     
  Partials        3        3              

Impacted Files	Coverage Δ
cwt/key_builder.py	96.96% <100.00%> (+0.04%)	⬆️
cwt/key_types/symmetric.py	98.73% <100.00%> (+2.47%)	⬆️

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