dkg: verify valIdx in frost dkg messages

[xenowits]

Adds check to ensure validator indexes (ValIdx) in frost DKG messages are in the range [0, num_validators).

Also refactors out broadcast and p2p callbacks and adds tests for them.

category: feature
ticket: #1888

[codecov]

Codecov Report

Patch coverage: 87.62% and project coverage change: +0.24 🎉

Comparison is base (617306e) 53.51% compared to head (be3b64a) 53.76%.

❗ Current head be3b64a differs from pull request most recent head 81574b8. Consider uploading reports for the commit 81574b8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2107      +/-   ##
==========================================
+ Coverage   53.51%   53.76%   +0.24%     
==========================================
  Files         171      172       +1     
  Lines       22252    22299      +47     
==========================================
+ Hits        11909    11989      +80     
+ Misses       8893     8871      -22     
+ Partials     1450     1439      -11     

Impacted Files	Coverage Δ
dkg/frostp2p.go	75.07% <87.50%> (+11.61%)	⬆️
dkg/dkg.go	59.38% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[corverroos]

Can we add a test?