Setting --initial-balance shouldn't fail if the subnet is bootstrapped in join

[adlrocha]

If subnet join is called with the --initial-balance flag after the subnet has already been spawned (or bootstrapped), setting the --initial-balance flag will cause the call to fail because once a subnet has been started validators are not allowed to set an initial balance.

The best way to go about it is that instead of failing when --initial-balance is set, when a subnet is bootstrapped instead of trying to call pre-fund, we should directly call fund` and send a top-down message to provide the initial balance.

We should also include docs on how to start a validator once a subnet has been already started (describing how they need to wait for a bottom-up checkpoint for the subnet to make the validator set effective)