fix Deneb RestPublishedSignedBlockContents REST JSON decoding

[tersec]

Found via

./hive --client-file ./configs/cancun.yaml --client go-ethereum,nimbus-bn,nimbus-vc --sim eth2/dencun --sim.limit "eth2-deneb-testnet/test-deneb-genesis" --sim.loglevel 5

after fixing #5519

VC logs of this issue:

DBG 2023-10-23 23:39:13.576+00:00 Got REST response headers from remote server remote=172.19.0.8:4000 request=/eth/v1/beacon/blocks status=400 http_method=POST
DBG 2023-10-23 23:39:13.576+00:00 Received REST response body from remote server remote=172.19.0.8:4000 request=/eth/v1/beacon/blocks contentType=application/json size=155
DBG 2023-10-23 23:39:13.577+00:00 Failed to deserialize REST JSON data       err="<data>(1, 47) Unexpected token 'tkCurlyLe' in place of 'array start bracket'" data="{\"code\":400,\"message\":\"Unable to decode data\",\"stacktraces\":{\"oResultPrivate\":true,\"vResultPrivate\":[\"\",\"<data>(1, 13445) Incorrect signed_block format\"]}}"
WRN 2023-10-23 23:39:13.577+00:00 Beacon node has incompatible configuration reason="Unable to decode error response: [Serialization error];400;publishBlock(first);invalid-request" node=http://172.19.0.8:4000[Nimbus/v23.10.0-7d102e-stateofus] node_index=0 node_roles=AGBSDT
WRN 2023-10-23 23:39:13.577+00:00 Unable to publish block                    reason="Unable to decode error response: [Serialization error];400;publishBlock(first);invalid-request" signingRoot=02946fd8 wall_slot=1 validator_index=78 blck="(slot: 1, proposer_index: 78, parent_root: \"c45ed64e\", state_root: \"b389964b\", eth1data: (deposit_root: d70a234731285c6804c2a4f56711ddb8c82c99740f207854891028af34e27e5e, deposit_count: 0, block_hash: 0e2dd0573112a2e7c3b47fcd350bd2e7fad74190cf400a59a4fe9be577f6e09a), graffiti: \"Nimbus/v23.10.0-7d102e-stateofus\", proposer_slashings_len: 0, attester_slashings_len: 0, attestations_len: 0, deposits_len: 0, voluntary_exits_len: 0, sync_committee_participants: 0, block_number: 1, fee_recipient: \"0x0000000000000000000000000000000000000000\", bls_to_execution_changes_len: 0, blob_kzg_commitments_len: 0)" service=block_service slot=1 validator=8376cabe block_root=c67166e6

BN logs:

DBG 2023-10-23 23:39:13.576+00:00 Failed to decode JSON data                 topics="rest_beaconapi" err="<data>(1, 13445) Incorrect signed_block format" data="{\"signed_block\":{\"message\":{\"slot\":\"1\",\"proposer_index\":\"78\",\"parent_root\":\"0xc45ed64e18affc40b6bd457da94edddded48e19d7c0a408872a0338291109d69\",\"state_root\":\"0xb389964bf50be75a47ad3184a88bd1a81c82debd0bfc8ad3026f32918aa2b678\",\"body\":{\"randao_reveal\":\"0x99b4d30c266a917d0a1d4dcaa184623da4e857c1b4f6af037f60b0d9c025aad5683c7ef186c9e92349f170130644c160125eee68f63874987666900e9910fe8ae22a253be446b95db72fcea232fc38b13055f954e7ea4dd089469c6b35d2f03f\",\"eth1_data\":{\"deposit_root\":\"0xd70a234731285c6804c2a4f56711ddb8c82c99740f207854891028af34e27e5e\",\"deposit_count\":\"0\",\"block_hash\":\"0x0e2dd0573112a2e7c3b47fcd350bd2e7fad74190cf400a59a4fe9be577f6e09a\"},\"graffiti\":\"0x4e696d6275732f7632332e31302e302d3764313032652d73746174656f667573\",\"proposer_slashings\":[],\"attester_slashings\":[],\"attestations\":[],\"deposits\":[],\"voluntary_exits\":[],\"sync_aggregate\":{\"sync_committee_bits\":\"0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000\",\"sync_committee_signature\":\"0xc00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000\"},\"execution_payload\":{\"parent_hash\":\"0x0e2dd0573112a2e7c3b47fcd350bd2e7fad74190cf400a59a4fe9be577f6e09a\",\"fee_recipient\":\"0x0000000000000000000000000000000000000000\",\"state_root\":\"0x...

[github-actions]

Unit Test Results

         9 files    1 092 suites   32m 25s ⏱️
  3 889 tests   3 592 ✔️ 297 💤 0 ❌
15 964 runs  15 641 ✔️ 323 💤 0 ❌

Results for commit b02d88b.