[EXT-BUILDER-API-01] Add the staked Builder API server path

[krisoshea-eth]

Scope rule

This is a Conditional parent package. Do not create child implementation issues or start work until the Week-19 selection rule activates it.

Entry criteria

• Core payload and signing model is stable.
• Specifications and existing Lodestar work are sufficiently settled.
• Maintainers request this package.
• One owner and one reviewer are named.

Current upstream input

builder-specs PR 165, beacon-APIs PR 630, and Lodestar PR 9594 are open, and the Lodestar team expects the specification and implementation work to settle over the next one to two weeks. Re-audit and reuse the landed shapes at activation, but do not pull DOMAIN_REQUEST_AUTH or request-signature verification into the core sidecar merely to anticipate this extension.

• Fix/request-auth-clarifications ethereum/builder-specs#165
• Update Beacon API for parity with Builder API and KeyManager API ethereum/beacon-APIs#630
• feat: implement gloas builder api ChainSafe/lodestar#9594

Candidate work

• Add bid endpoint, preferences/auth, and settled request-auth verification.
• Accept the required signed-block input.
• Reuse the core payload/cache adapter.
• Add bounds and conformance/interoperability tests.

Evidence target

The settled staked Builder API uses the same payload/cache implementation and passes bounded interoperability evidence.

Stop rule

Do not create a second payload or cache implementation.

Activation record

• Decision:
• Owner:
• Reviewer:
• Bounded child issues:
• Result:

[linear-code]

LOD-32