Add CL light client support

[etan-status]

There is an additional nimbus_light_client binary available in nimbus-eth2 unstable (and, eventually, also on the other branches). It would be interesting to have a few instances of these in the fleet to collect metrics regarding bandwidth (primary), memory, and CPU.

Compared to nimbus_beacon_node, the nimbus_light_client binary does not need a data-dir.
It takes similar arguments for configuring logs and libp2p settings (port numbers etc).

However, there is an additional --trusted-block-root argument that needs to be initialized to a recent checkpoint block root.

To obtain a --trusted-block-root, an existing nimbus_beacon_node could be queried:

curl -s "http://hostname:port/eth/v1/beacon/headers/finalized" | jq -r '.data.root'

For all the launch arguments, see https://github.com/status-im/nimbus-eth2/blob/62bfe97bbed8acd511817ef85d6ff9acaebeefe6/beacon_chain/conf_light_client.nim

For sample integration into the launch_local_testnet script used in CI, see https://github.com/status-im/nimbus-eth2/blob/62bfe97bbed8acd511817ef85d6ff9acaebeefe6/scripts/launch_local_testnet.sh#L742-L752