SSE for payload attributes (#305)

* SSE for payload attributes

* Tweak encoding wording

* Add comment about beacon node config

apis/eventstream/index.yaml

@@ -32,6 +32,7 @@ get:
             - contribution_and_proof
             - light_client_finality_update
             - light_client_optimistic_update
+            - payload_attributes
   responses:
     "200":
       description: Opened SSE stream.
@@ -91,6 +92,36 @@ get:
               value: |
                 event: light_client_optimistic_update
                 data: {"version":"phase0", "data": {"attested_header": {"beacon": {"slot":"1", "proposer_index":"1", "parent_root":"0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2", "state_root":"0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2", "body_root":"0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2"}}, "sync_aggregate": {"sync_committee_bits":"0x01", "sync_committee_signature":"0x1b66ac1fb663c9bc59509846d6ec05345bd908eda73e670af888da41af171505cc411d61252fb6cb3fa0017b679f8bb2305b26a285fa2737f175668d0dff91cc1b66ac1fb663c9bc59509846d6ec05345bd908eda73e670af888da41af171505"}, "signature_slot":"1"}}
+            payload_attributes:
+              description: |
+                The node has computed new payload attributes for execution payload building.
+
+                This event gives block builders and relays sufficient information to construct or
+                verify a block at `proposal_slot`. The meanings of the fields are:
+
+                - `version`: the identifier of the beacon hard fork at `proposal_slot`, e.g.
+                  `"bellatrix"`, `"capella"`.
+                - `proposal_slot`: the slot at which a block using these payload attributes may be
+                   built.
+                - `parent_block_root`: the beacon block root of the parent block to be built upon.
+                - `parent_block_number`: the execution block number of the parent block.
+                - `parent_block_hash`: the execution block hash of the parent block.
+                - `proposer_index`: the validator index of the proposer at `proposal_slot` on
+                   the chain identified by `parent_block_root`.
+                - `payload_attributes`: beacon API encoding of `PayloadAttributesV<N>` as
+                   defined by the `execution-apis` specification. The version `N` must match the
+                   payload attributes for the hard fork matching `version`.
+                   The beacon API encoded object must have equivalent fields to its counterpart in
+                   `execution-apis` with two differences: 1) `snake_case` identifiers must be used
+                   rather than `camelCase`; 2) integers must be encoded as quoted decimals rather
+                   than big-endian hex.
+
+                The frequency at which this event is sent may depend on beacon node configuration.
+                The fee recipient may also be set via beacon node config, but should likely be
+                ignored by block builders and most other API consumers.
+              value: |
+                event: payload_attributes
+                data: {"version": "capella", "data": {"proposer_index": "123", "proposal_slot": "10", "proposal_block_number": "9", "parent_block_root": "0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2", "parent_block_hash": "0x9a2fefd2fdb57f74993c7780ea5b9030d2897b615b89f808011ca5aebed54eaf", "payload_attributes": {"timestamp": "123456", "prev_randao": "0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2", "suggested_fee_recipient": "0x0000000000000000000000000000000000000000", "withdrawals": [{"index": "5", "validator_index": "10", "address": "0x0000000000000000000000000000000000000000", "amount": "15640"}]}}},
     "400":
       description: "The topics supplied could not be parsed"
       content: