eth2exp: add CalculateCommitteeSubscriptionResponse function

[xenowits]

Adds CalculateCommitteeSubscriptionResponse function to eth2exp/attagg.go that calculates is_aggregator and returns a BeaconCommitteeSubscriptionResponse.

category: feature
ticket: #1091

[codecov]

Codecov Report

Merging #1098 (d362841) into main (46665e2) will decrease coverage by 0.26%.
The diff coverage is 56.71%.

❗ Current head d362841 differs from pull request most recent head 68bf938. Consider uploading reports for the commit 68bf938 to get more accurate results

@@            Coverage Diff             @@
##             main    #1098      +/-   ##
==========================================
- Coverage   53.51%   53.25%   -0.27%     
==========================================
  Files         127      128       +1     
  Lines       14348    14486     +138     
==========================================
+ Hits         7679     7715      +36     
- Misses       5571     5665      +94     
- Partials     1098     1106       +8     

Impacted Files	Coverage Δ
testutil/beaconmock/beaconmock.go	59.34% <0.00%> (-2.73%)	⬇️
testutil/beaconmock/server.go	54.47% <ø> (ø)
eth2util/eth2exp/attagg.go	22.58% <52.83%> (ø)
eth2util/hash.go	88.00% <100.00%> (+8.00%)	⬆️
core/scheduler/scheduler.go	73.49% <0.00%> (-0.55%)	⬇️

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[corverroos]

Don't reinvent the wheel, use eth2client

[xenowits]

Don't reinvent the wheel, use eth2client

Yeah sorry, my bad smh