feat: add client side max connection age for http/2

[pjfanning]

Server side is #1316.

Online search says Akka has added these configs but Pekko has no equivalent.

Useful for better load balancing in clusters. Connections can be pinned to existing server instances and new instances can be underutilized if we don't encourage clients to seek new connections from time to time.

akka.http.client.http2 {
  persistent-connection-max-age = 10m      # 0s = disabled (default)
  persistent-connection-max-requests = 0   # also available
}

We should implement a clean room Pekko solution without looking at Akka's impl due to licensing issues.

[Rayan-and-beyond]

I can work on this. I’ll implement the client-side HTTP/2 connection age limit with focused coverage for retirement/reconnect behavior and update the client configuration docs.