[BFT] model.Proposal refactoring

[durkmurder]

#6517

Context

This PR implements refactoring of model.Proposal, all details are in attached issue. This PR very closes follows proposed solution.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.10619% with 27 lines in your changes missing coverage. Please review.

Project coverage is 41.45%. Comparing base (c1a1cc0) to head (78dd10d).

Files with missing lines	Patch %	Lines
consensus/recovery/recover.go	20.00%	4 Missing ⚠️
consensus/hotstuff/mocks/vote_aggregator.go	0.00%	3 Missing ⚠️
consensus/hotstuff/mocks/validator.go	0.00%	2 Missing ⚠️
consensus/hotstuff/mocks/vote_processor_factory.go	50.00%	0 Missing and 2 partials ⚠️
consensus/hotstuff/mocks/event_handler.go	50.00%	0 Missing and 1 partial ⚠️
consensus/hotstuff/mocks/event_loop.go	0.00%	1 Missing ⚠️
consensus/hotstuff/mocks/participant_consumer.go	0.00%	1 Missing ⚠️
consensus/hotstuff/mocks/safety_rules.go	75.00%	0 Missing and 1 partial ⚠️
...sensus/hotstuff/mocks/vote_aggregation_consumer.go	0.00%	1 Missing ⚠️
...stuff/mocks/vote_aggregation_violation_consumer.go	0.00%	1 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6526      +/-   ##
==========================================
+ Coverage   41.28%   41.45%   +0.17%     
==========================================
  Files        2050     2022      -28     
  Lines      182574   181034    -1540     
==========================================
- Hits        75375    75049     -326     
+ Misses     100870    99711    -1159     
+ Partials     6329     6274      -55     

Flag	Coverage Δ
unittests	41.45% <76.10%> (+0.17%)	⬆️

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[AlexHentschel]

very cool. Thanks for the cleanup. Mostly minor comments regarding documentation.