Set fee recipient validator index as per spec. (#10814)

The beacon API specification states that the `validator_index` field of
the `fee_recipient` structure is a quoted string rather than a bare
integer.  This fixes up the encoding to match the spec.

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

beacon-chain/rpc/apimiddleware/structs.go

@@ -550,7 +550,7 @@ type indexedAttestationJson struct {
 }
 
 type feeRecipientJson struct {
-	ValidatorIndex uint64 `json:"validator_index"`
+	ValidatorIndex string `json:"validator_index"`
 	FeeRecipient   string `json:"fee_recipient" hex:"true"`
 }