Add attestation preaggregation

[meyer9]

I'd like to allow the option for multiple domains to be aggregated, which should be possible, but the interface needs to be updated. That way we can verify 1 signature when syncing an epoch instead of 3.

[codecov-io]

Codecov Report

Merging #51 into master will decrease coverage by 0.19%. The diff coverage is 12.12%.

@@            Coverage Diff            @@
##           master      #51     +/-   ##
=========================================
- Coverage   36.02%   35.83%   -0.2%     
=========================================
  Files          20       20             
  Lines        3375     3416     +41     
=========================================
+ Hits         1216     1224      +8     
- Misses       2023     2056     +33     
  Partials      136      136

Impacted Files	Coverage Δ
beacon/blockchain.go	45.27% <ø> (ø)	:arrow_up:
beacon/mempool.go	0% <ø> (ø)	:arrow_up:
beacon/state.go	61.6% <ø> (ø)	:arrow_up:
beacon/db/badger.go	0% <ø> (ø)	:arrow_up:
beacon/db/memory.go	26.47% <ø> (ø)	:arrow_up:
beacon/syncmanager.go	0% <0%> (ø)	:arrow_up:
primitives/state.go	15.55% <0%> (-0.02%)	:arrow_down:
beacon/statemanager.go	81.02% <100%> (+1.17%)	:arrow_up:

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[meyer9]

I'll actually do the multi-domain verification function in another PR because it's not strictly required for the testnet.