block production: wire Gloas builder bid fetch and notify

[NikhilSharmaWe]

Part of #769
Depends on #819.

Summary

Wires the Gloas Staked Builder API into block production:

• During propose, fetch an execution payload bid over HTTP in parallel with local payload prep, validate it through fork choice (on_api_execution_payload_bid), then select the highest-value bid among accepted HTTP and gossip bids (self-build if none).
• Optionally sign RequestAuth with local keys when a bid fetch will run.
• After gossiping the signed beacon block, notify the builder via submitSignedBeaconBlock for external builders (best-effort). Accepted HTTP bids are also re-gossiped so peers can see them before the block.

Behavior

• Pre-Gloas path is unchanged (blinded header / post_blinded_block).
• On Gloas, gossip remains the primary bid source; HTTP fetch is an additional path for builders that expose the Staked Builder API.
• Ordering for external builder proposals: sign → on_own_block → gossip beacon block → HTTP notify.
• submitSignedBeaconBlock is notify-only (202); the proposer still gossips the block.
• Web3Signer does not support RequestAuth yet; those validators fetch without auth (logged at info).
• HTTP preferences (submitBuilderPreferences) and skipping register_validators at Gloas are follow-ups.

Test plan

• cargo test -p builder_api --features blst
• cargo test -p signer -p kzg_utils --features bls/blst,kzg_utils/blst request_auth
• On a Gloas setup with --builder-api-url: confirm HTTP 200 bids are selected and the builder is notified after gossip; 204 and invalid bids fall back to gossip/self-build; self-build skips HTTP fetch and notify; the block is gossiped regardless of notify result.