grpc: Add keepalive Support

[Jozott00]

Subsystem
gRPC

Problem Description
To reduce latency because of required reconnections to the server, the client sometimes wants to keep a connection open.

Solution
This can be done by setting the keepalive configuration option on the gRPC ClientConfiguration.

GrpcClient("localhost", 50051) {
    keepAlive {
         time = 10.seconds
         timeout = 20.seconds
         withoutCalls = false
    }
}