add initial electra spec (#101)

* add initial electra spec

* move indirectly updated types into note

* adjust for move of exec reqs from exec payload to beacon block

* use electra constants for max lengths of attester slasings & attestations

* replace execution requests with corresponding root

* add ExecutionBundle and extend BuilderBid

* clean up execution bundle

* add notes to accompany type changes

* fix broken link

* replace exec reqs root with exec reqs

* remove execution bundle container

specs/electra/builder.md

@@ -0,0 +1,47 @@
+# Electra -- Builder Specification
+
+## Introduction
+
+This is the modification of the builder specification accompanying the Electra upgrade.
+
+The behavior defined by the specification is consistent with previous forks except for the changes to the types given below.
+
+## Containers
+
+### Extended containers
+
+#### `BuilderBid`
+
+Note: `SignedBuilderBid` is updated indirectly.
+
+```python
+class BuilderBid(Container):
+    header: ExecutionPayloadHeader
+    blob_kzg_commitments: List[KZGCommitment, MAX_BLOB_COMMITMENTS_PER_BLOCK]
+    execution_requests: ExecutionRequests # [New in Electra]
+    value: uint256
+    pubkey: BLSPubkey
+```
+
+#### `BlindedBeaconBlockBody`
+
+Note: `BlindedBeaconBlock` and `SignedBlindedBeaconBlock` types are updated indirectly.
+
+```python
+class BlindedBeaconBlockBody(Container):
+    randao_reveal: BLSSignature
+    eth1_data: Eth1Data
+    graffiti: Bytes32
+    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
+    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS_ELECTRA] # [Modified in Electra:EIP7549]
+    attestations: List[Attestation, MAX_ATTESTATIONS_ELECTRA] # [Modified in Electra:EIP7549]
+    deposits: List[Deposit, MAX_DEPOSITS]
+    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
+    sync_aggregate: SyncAggregate
+    execution_payload_header: ExecutionPayloadHeader
+    bls_to_execution_changes: List[SignedBLSToExecutionChange, MAX_BLS_TO_EXECUTION_CHANGES]
+    blob_kzg_commitments: List[KZGCommitment, MAX_BLOB_COMMITMENTS_PER_BLOCK]
+    execution_requests: ExecutionRequests # [New in Electra]
+```
+
+[execution-payload-and-blobs-bundle-deneb]: ../deneb/builder.md#executionpayloadandblobsbundle