Improve SYMKey init (allow setting k and/or key)

[jschlyter]

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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.44%. Comparing base (e95fb93) to head (a72c2b4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cryptojwt/jwk/hmac.py	83.33%	0 Missing and 1 partial ⚠️
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #197   +/-   ##
=======================================
  Coverage   76.43%   76.44%           
=======================================
  Files          44       44           
  Lines        4550     4556    +6     
  Branches      859      862    +3     
=======================================
+ Hits         3478     3483    +5     
  Misses        785      785           
- Partials      287      288    +1     

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