Passivation configuration by the discovery protocol

[marcellanz]

With cloudstateio/cloudstate#486 entity passivation can be configured through the discovery protocol.

//
// The semantics is to provide a flexible way for entity user functions to configure the passivation strategy.
// This strategy is sent to the proxy at discovery time allowing the proxy to configure the corresponding entities.
// The only passivation strategy supported is the timeout strategy and configuring this is optional for the entity.
// If an entity user function does not configure the passivation strategy the proxy used its fallback default value.
//
// The passivation strategy for the entity user function.
message EntityPassivationStrategy {
    oneof strategy {
        // the timeout passivation strategy.
        TimeoutPassivationStrategy timeout = 1;
    }
}