Feat: add new --advertise-false-custody-group-count cli flag for PeerDAS testing

[startup-dreamer]

Issue Addressed

Closes: #6973

Proposed Changes

This PR adds a new CLI flag --advertise-false-custody-group-count that allows nodes to advertise a false custody group count in their metadata for PeerDAS testing purposes. This enables testing scenarios where nodes need to mislead peers about their custody group count without altering the actual RPC logic.

Additional Info

When this flag is set:

• If PeerDAS is scheduled, the node will advertise the specified custody group count in its metadata.
• If the specified count is outside the valid range (<= number_of_custody_groups), the node will log a warning and use the normal count.
• If PeerDAS is not scheduled, the flag has no effect.

Future Work

This flag is intended to be temporary and will be removed before the PeerDAS release, as indicated by the TODO comment.

[mergify]

This pull request has merge conflicts. Could you please resolve them @startup-dreamer? 🙏

[dapplion]

Does this affect both the ENR value and the value returned in the Status ReqResp message?

[startup-dreamer]

ENR is built with the metadata (false custody_group_count if provided) and The metadata returned in response to METADATA requests (which is part of the ReqResp protocol) so I'd say yes it affects both.

[dapplion]

If the user restarts the node with a different value of the false_count flag, will we bump the seq_number of the ENR?

[startup-dreamer]

I rechecked I don't think it bumps the seq_number of ENR, Since the ENR is built from the chain spec and the
subscribe_all_data_column_subnets, not from the false count flag (sorry for prev comment I am still getting familiar with the codebase). Does this make sense @dapplion

lighthouse/beacon_node/lighthouse_network/src/discovery/enr.rs

Lines 256 to 264 in bf955c7

	// only set `csc` if PeerDAS fork epoch has been scheduled
	if spec.is_peer_das_scheduled() {
	let custody_subnet_count = if config.subscribe_all_data_column_subnets {
	spec.data_column_sidecar_subnet_count
	} else {
	spec.custody_requirement
	};
	builder.add_value(PEERDAS_CUSTODY_SUBNET_COUNT_ENR_KEY, &custody_subnet_count);
	}

[startup-dreamer]

But I think I have to give more time to understand this properly.

[dapplion]

If the user restarts the node with a different value of the false_count flag, will we bump the seq_number of the ENR?