Add bid proofs to the builder bid response

specs/builder.md

@@ -31,6 +31,12 @@
 | - | - |
 | `DOMAIN_APPLICATION_BUILDER` | `DomainType('0x00000001')` |
 
+### Execution proof
+
+| Name | Value |
+| - | - |
+| `MAX_EXECUTION_PROOF_DEPTH` | `256/4` (= 64) |
+
 ## Containers
 
 Consider the following definitions supplementary to the definitions in
@@ -76,6 +82,8 @@ class BuilderBid(Container):
     header: ExecutionPayloadHeader
     value: uint256
     pubkey: BLSPubkey
+    fee_recipient: ExecutionAddress
+    proof: List[Bytes32, MAX_EXECUTION_PROOF_DEPTH]
 ```
 
 ##### `SignedBuilderBid`

types/bellatrix/bid.yaml

@@ -12,6 +12,12 @@ Bellatrix:
       pubkey:
         $ref: "../../beacon-apis/types/primitive.yaml#/Pubkey"
         description: "BLS public key of builder."
+      fee_recipient:
+        $ref: '../../beacon-apis/types/primitive.yaml#/ExecutionAddress'
+        description: "The address the bid will be paid to, as registered previously with builder"
+      proof:
+        $ref: "../proof.yaml#/Proof"
+        description: "Merkle proof of post balance of the `fee_recipient` account"
 
   SignedBuilderBid:
     type: object

types/proof.yaml

@@ -0,0 +1,6 @@
+Proof:
+  type: array
+  items:
+    allOf:
+      - $ref: "../beacon-apis/types/primitive.yaml#/Bytes32"
+  maxItems: 64