validator: take the default target gas limit from GAS_LIMIT_SCHEDULE #795

The validator client's default target gas limit is a fixed 60000000 (DEFAULT_GAS_LIMIT, overridden by --gas-limit or per validator through the keymanager API). From gloas it is signed into every proposer preference, and produceBlockV4 builds this client's own payload toward the preference's target_gas_limit. A validator left on the default therefore pulls the gas limit toward 60M on every gloas proposal, whatever the network targets.

Two built-in networks schedule something else in their config.yaml's GAS_LIMIT_SCHEDULE:

• Sepolia: 200000000 from epoch 353024 (bin/ethlambda/assets/sepolia/config.yaml).
• Platåberget: 200000000 from epoch 1566 (bin/ethlambda/assets/plataberget/config.yaml).

Lighthouse takes its default from the network config's GAS_LIMIT_SCHEDULE before falling back to a built-in value. This client cannot: the beacon node parses the key into nothing (it is one of the keys the ignored-keys warning names), and /eth/v1/config/spec does not serve it.

What it takes

• Beacon node: parse GAS_LIMIT_SCHEDULE into Config and serve it from /eth/v1/config/spec. Config is stored SSZ-encoded under Metadata["config"], so a new field there is a DB_VERSION bump unless it is kept out of the stored struct.
• Validator client: when --gas-limit is not given, default to the scheduled limit in force at the proposal's epoch, read from /config/spec, and fall back to 60000000 for a node that does not serve the key.

Until this lands, docs/cli.md and the README tell operators on Sepolia and Platåberget to pass --gas-limit 200000000.

Raised in the review of #707.