cli/net_params: Warn on empty public-addr when starting a validator node (#5240)

This PR shows a warning when the `--public-addr` is not provided for
validators.

In the future, we'll transform this warning into a hard failure.
Validators are encouraged to provide this parameter for better
availability over the network.

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <[email protected]>