core: add verification for sync selection

[dB2510]

Implement core.Eth2SignedData for SyncCommitteeSelection to support signature verification.

category: feature
ticket: #1264

[codecov]

Codecov Report

Base: 54.03% // Head: 54.18% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (2169d18) compared to base (1b09101).
Patch coverage: 44.44% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1373      +/-   ##
==========================================
+ Coverage   54.03%   54.18%   +0.14%     
==========================================
  Files         144      144              
  Lines       17507    17516       +9     
==========================================
+ Hits         9460     9491      +31     
+ Misses       6749     6726      -23     
- Partials     1298     1299       +1     

Impacted Files	Coverage Δ
core/signeddata.go	46.74% <0.00%> (+3.88%)	⬆️
core/eth2signeddata.go	80.95% <100.00%> (+1.29%)	⬆️
app/peerinfo/peerinfo.go	54.48% <0.00%> (-3.45%)	⬇️
core/qbft/qbft.go	81.97% <0.00%> (+0.85%)	⬆️
app/vmock.go	75.40% <0.00%> (+1.06%)	⬆️

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