feat: Introduce cutoff threshold for endorsement ratio

[tayfunelmas]

Issue: #11900.

This PR introduces a cutoff threshold for chunk endorsement ratio. We use the same kickout threshold as the cutoff threshold. It is currently 80, but we will make it 70 when stabilizing this feature.

If the endorsement ratio is less than the cutoff ratio, it is treated 0, otherwise treated 1, when computing the average uptime ratio (including block and chunk production and endorsement).

For this, we introduce a new struct ValidatorOnlineThresholds to contain online_min_threshold and online_max_threshold as well as endorsement_cutoff_threshold (initialized to the kickout threshold=70 if feature ChunkEndorsementsInBlockHeader is enabled). ValidatorOnlineThresholds is initialized from EpochConfig. We pass this struct to calculate_rewards, which then calls get_validator_online_ratio to apply the cutoff if present.

We performed simulation of last 5 epochs in mainnet, results are in this doc. Only the chunk validators with very low endorsement ratio are kicked out and do not get any reward.

Testing: We added some unittests for the basic logic. We will later add integration tests for the full behavior.

[codecov]

Codecov Report

Attention: Patch coverage is 93.61702% with 18 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (3718df6) to head (1197e10).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
chain/epoch-manager/src/lib.rs	38.88%	10 Missing and 1 partial ⚠️
core/primitives/src/epoch_manager.rs	41.66%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12047      +/-   ##
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+ Coverage   71.40%   71.45%   +0.04%     
==========================================
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  Lines      163936   164177     +241     
  Branches   163936   164177     +241     
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+ Hits       117055   117305     +250     
+ Misses      41754    41744      -10     
- Partials     5127     5128       +1     

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[tayfunelmas]

moved these to the ValidatorOnlineThresholds struct passed to calculate_reward.
In fact we do not re-initialize the RewardCalculator when there is a new epoch or protocol change, so this way it is safer as we will need to change the online thresholds (endorsement cutoff in this case) via protocol upgrades.