cli: Ensure chainSpec's bootnodes are still reachable on the p2p network

[lexnv]

The correctness of any light-client depends heavily on the list of provided bootnodes.

Establish a connection on the p2p network with boot nodes to ensure they are still alive.

The following protocols should be probed:

• ping: Check if bootnode is reachable
• identify: The bootnode must support protocols that align with the fork/genesis_hash specified in the spec

Should be done after the following is closed:

• cli: Command to fetch chainSpec and optimise its size #1263