Unstaked Builder API

[bharath-123]

This PR extends the Builder-API for ePBS to support unstaked builders - builders who do not have staked collateral on the beacon chain.

Background

While the staked builder flow (introduced in the parent PR) allows builders with on-chain collateral to provide SignedExecutionPayloadBid directly to proposers, unstaked builders operate differently. They function similarly to current MEV-Boost relays, providing full block contents while utilizing the new ePBS primitives.
The key distinction is that unstaked builders use BUILDER_INDEX_SELF_BUILD (2**64 - 1) as their builder index, indicating that the proposer is effectively "self-building" with the unstaked builder's block contents.

New APIs

We introduce 2 new APIs for unstaked builder interactions:

1. getBuilderBid: GET /eth/v1/builder/header/{slot}/{parent_hash}/{parent_root}/{pubkey}
   This API is called by the proposer to an unstaked builder/relay. The builder returns a SignedBuilderBid containing: ExecutionPayloadHeader for the proposed block blob_kzg_commitments for any associated blobs, execution_requests and SignedExecutionPayloadBid with builder_index set to BUILDER_INDEX_SELF_BUILD.
2. submitBlockAndEnvelope: POST /eth/v1/builder/block_and_envelope
   This API is called by the proposer to submit both the SignedBeaconBlock and a SignedBlindedExecutionPayloadEnvelope to the unstaked builder. Upon receiving this, the builder validates the block and envelope signatures, unblinds the BlindedExecutionPayloadEnvelope with the full execution payload, broadcasts the SignedExecutionPayloadEnvelope to the PTC committee

Flow Overview

1. Proposer calls getBuilderBid → Unstaked Builder
2. Unstaked Builder returns SignedBuilderBid (header, commitments, requests, bid)
3. Proposer constructs SignedBeaconBlock with the bid
4. Proposer constructs SignedBlindedExecutionPayloadEnvelope
5. Proposer calls submitBlockAndEnvelope with both objects → Unstaked Builder
6. Unstaked Builder unblinds the envelope and broadcasts SignedExecutionPayloadEnvelope to PTC committee

Relationship to Staked Builder API

This specification complements the staked builder API. Proposers can interact with both staked and unstaked builders simultaneously, choosing the best bid from either source. The builder_index field distinguishes between them:
Staked builders: Use their actual on-chain builder index
Unstaked builders: Use BUILDER_INDEX_SELF_BUILD

Disclaimer: This specification was generated using the help of AI. This specification has been drafted to understand how the Builder-API for unstaked builders could potentially look like. The author strongly discourages unstaked builders to encourage with the protocol as the staking amount for builders has gone down to 1Eth.