EIP-7549: Append new committee_bits field to end of Attestation

Introducing new fields in the middle of an existing `Container`
pointlessly breaks merkleization of all subsequent fields.
In the case of `committee_bits`, it is also misleading, as
`signature` only covers `data` inside `Attestation`.

Author: etan-status

specs/electra/beacon-chain.md

@@ -287,8 +287,8 @@ class AttesterSlashing(Container):
 class Attestation(Container):
     aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE * MAX_COMMITTEES_PER_SLOT]  # [Modified in Electra:EIP7549]
     data: AttestationData
-    committee_bits: Bitvector[MAX_COMMITTEES_PER_SLOT]  # [New in Electra:EIP7549]
     signature: BLSSignature
+    committee_bits: Bitvector[MAX_COMMITTEES_PER_SLOT]  # [New in Electra:EIP7549]
 ```
 
 #### `IndexedAttestation`