Add Channel State API about akka-grpc

[johanandren]

I haven't really dug into what would be required/is easily doable, but instead of adding such a low level api, I wonder if we could have an eager connect client setting. Or do you think it would be important to also be able to imperatively query-trigger connect later during the lifecycle of a client?

[enforser]

Eager connect sounds great and would suit my use case perfectly. In fact I think it would be fairly straightforward to add that.

It would just be calling .getState(true) on the ManagedChannel returned here: https://github.com/akka/akka-grpc/blob/main/runtime/src/main/scala/akka/grpc/internal/NettyClientUtils.scala#L112-L126