[Feature Request] Add client keep-alive support

[cretz]

Describe the solution you'd like

We need to support client keep alive. Up for debate:

• Whether it remains off by default and opt-in or we turn it on by default w/ a fairly high value
  • I'm leaning to on by default (EDIT: Seems it was decided to turn on by default in https://github.com/temporalio/features/issues/123)
• Whether to use TCP keep alive or HTTP/2 keep alive
  • I'm leaning HTTP/2 keep alive, it's the higher level gRPC one that does a better job and is more configurable
• Which options to expose to users
  • I'm leaning to interval, timeout, and while-idle