Redesign of the attestation pool (#14324)

* 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

CHANGELOG.md

@@ -14,7 +14,8 @@ The format is based on Keep a Changelog, and this project adheres to Semantic Ve
 - `Finished building block`: Display error only if not nil.
 - Added support to update target and max blob count to different values per hard fork config.
 - Log before blob filesystem cache warm-up.
-- 
+- New design for the attestation pool. [PR](https://github.com/prysmaticlabs/prysm/pull/14324)
+
 ### Changed
 
 - Process light client finality updates only for new finalized epochs instead of doing it for every block.