WIP: crypto/keys: detach DecodeSignature from baseKeybase struct

[alessio]

It makes more sense for DecodeSignature() to be a standalone function.

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[codecov]

Codecov Report

Merging #5838 into master will increase coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #5838      +/-   ##
==========================================
+ Coverage   32.33%   32.33%   +<.01%     
==========================================
  Files         353      353              
  Lines       39475    39477       +2     
==========================================
+ Hits        12763    12765       +2     
  Misses      25439    25439              
  Partials     1273     1273

Impacted Files	Coverage Δ
crypto/keys/keyring.go	52.08% <100%> (+0.16%)	⬆️
crypto/keys/keybase.go	66.31% <100%> (+0.35%)	⬆️
crypto/keys/keybase_base.go	72.51% <50%> (ø)	⬆️
crypto/keys/lazy_keybase.go	56.89% <88.88%> (-0.37%)	⬇️

[alessio]

Superseded, see #5844