Add support for key derivation without kid.

[dajiaji]

No description provided.

[codecov]

Codecov Report

Merging #120 (a5f2d38) into main (124df5a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #120   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          25       25           
  Lines        2068     2069    +1     
  Branches      521      522    +1     
=======================================
+ Hits         2060     2061    +1     
  Misses          6        6           
  Partials        2        2           

Impacted Files	Coverage Δ
cwt/recipient_interface.py	100.00% <ø> (ø)
cwt/recipient_algs/ecdh_direct_hkdf.py	100.00% <100.00%> (ø)
cwt/recipients.py	100.00% <100.00%> (ø)

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