Prefer raw key to FernetEncrypter

[jschlyter]

If a pre-derived key is supplied to FernetEncrypter, prefer that to password/salt.

[codecov-commenter]

Codecov Report

Merging #122 (1723845) into main (d172218) will increase coverage by 0.17%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   77.26%   77.44%   +0.17%     
==========================================
  Files          41       42       +1     
  Lines        4324     4362      +38     
  Branches      836      842       +6     
==========================================
+ Hits         3341     3378      +37     
  Misses        718      718              
- Partials      265      266       +1     

Impacted Files	Coverage Δ
src/cryptojwt/jwe/fernet.py	97.36% <97.36%> (ø)
src/cryptojwt/jws/jws.py	80.87% <100.00%> (ø)

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