* Rename existing AttestationCache
* Cache with simple Add
* fix import cycle
* logic for unaggregated, aggregated and block attestations
* some small fixes
* remove Seen
* finishing touches
* feature flag
* extract forkchoice atts to separate type
* gate new functionality behind feature flag
* revert test files
* preparing for review
* change Id to [32]byte
* Potuz's review
* Potuz's review pt 2
* Nishant's review
* keep flat list of atts
* fix ForkchoiceAttestations() function
* Tests for Add, GetAll, Count
* Tests for remaining functions
* use DeepEqual
* fix tests
* documentation
* changelog <3
* v2 handlers
* nil check for forkchoice atts
* guard against 0 bits set
* fix failing test
* Preston's review
* better godocs