Add new metadatav3 field custody_subnet_count to GET node/identity API

[jimmygchen]

Description

The custody_subnet_count field has been added to MetaDataV3, but we don't return it in the HTTP api yet and should be added:

lighthouse/beacon_node/http_api/src/lib.rs

Lines 2897 to 2913 in ae83901

	metadata: api_types::MetaData {
	seq_number: *meta_data.seq_number(),
	attnets: format!(
	"0x{}",
	hex::encode(meta_data.attnets().clone().into_bytes()),
	),
	syncnets: format!(
	"0x{}",
	hex::encode(
	meta_data
	.syncnets()
	.cloned()
	.unwrap_or_default()
	.into_bytes()
	)
	),
	},