Refactor validation logic into a mutex-free pluggable struct

[masih]

Refactor the validation logic out of Participant and into its own dedicated struct that is mutex-free and listens to the progress made by the participant to infer the correct validation path.

The change above significantly reduces the need for mutex control over current instance, which makes it easier to plug in extra conditional behaviour, e.g. #583.

Fixes #561

[codecov]

Codecov Report

Attention: Patch coverage is 89.50276% with 19 lines in your changes missing coverage. Please review.

Project coverage is 76.72%. Comparing base (f5af3ff) to head (9fad427).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
gpbft/validator.go	90.84%	7 Missing and 7 partials ⚠️
gpbft/options.go	20.00%	4 Missing ⚠️
gpbft/participant.go	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #657      +/-   ##
==========================================
- Coverage   76.90%   76.72%   -0.18%     
==========================================
  Files          64       65       +1     
  Lines        5451     5487      +36     
==========================================
+ Hits         4192     4210      +18     
- Misses        857      872      +15     
- Partials      402      405       +3     

Files with missing lines	Coverage Δ
gpbft/gpbft.go	88.18% <100.00%> (+0.13%)	⬆️
internal/caching/grouped_cache.go	92.30% <100.00%> (-3.53%)	⬇️
gpbft/participant.go	84.35% <87.50%> (-3.51%)	⬇️
gpbft/options.go	62.71% <20.00%> (-3.96%)	⬇️
gpbft/validator.go	90.84% <90.84%> (ø)

... and 4 files with indirect coverage changes