Adding validator to primary network should not be triggered if the validator is pending

[Nuttymoon]

There is a minimum delay of 5min between the moment the platform.addValidator transaction is accepted and the validator is considered as validating the primary network.

When running ash.avalanche.add_validator, we should also check if the validator is pending with platform.getPendingValidators additionally to checking if it is currently validating (with platform.getCurrentValidators).