[Altair] Sync committee pools (#2321)

Add pools supporting sync committees:
- naive sync aggregation pool
- observed sync contributions pool
- observed sync contributors pool
- observed sync aggregators pool

Add SSZ types and tests related to sync committee signatures.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: realbigsean <seananderson33@gmail.com>

Cargo.lock

@@ -4429,6 +4429,7 @@ name = "operation_pool"
 version = "0.2.0"
 dependencies = [
  "beacon_chain",
+ "derivative",
  "eth2_ssz",
  "eth2_ssz_derive",
  "int_to_bytes",
@@ -4442,6 +4443,7 @@ dependencies = [
  "serde_derive",
  "state_processing",
  "store",
+ "superstruct",
  "types",
 ]