Remove change event signer

[frnandu]

since the signer is injected into many usecases, we cannot later replace it without causing problems. So if client needs to change the signer, it should create a new instance of ndk

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.71%. Comparing base (b33b78e) to head (2d64600).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   63.72%   63.71%   -0.01%     
==========================================
  Files          59       59              
  Lines        2172     2169       -3     
==========================================
- Hits         1384     1382       -2     
+ Misses        788      787       -1     

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