initial simple/naive attestation aggregation

[tersec]

#497

This is mostly the aggregation per se.

Remaining elements, intentionally not in this PR:

• calling this from beacon_node in time for 2/3 through slot durations; this should be fast enough that doing so at 2/3 through suffices;
• adding/double-checking validations some of which already exist in attestation_pool
• now that Integrate the native LibP2P implementation #632 has been merged, use it for the various validations on the message level, mostly max gossip message size and matching topics/message types.
• tests for validation of those messages (dependent on mocking gossipsub messages)

[tersec]

https://ci.status.im/blue/organizations/jenkins/nimbus%2Fnim-beacon-chain%2Fprs/detail/PR-629/6/pipeline/68 failure looks spurious:

+ make -j6 DISABLE_TEST_FIXTURES_SCRIPT=1 test
mkdir: cannot create directory ‘build’: File exists
vendor/nimbus-build-system/makefiles/targets.mk:17: recipe for target 'build' failed
make: *** [build] Error 1
make: *** Waiting for unfinished jobs....
script returned exit code 2