Keepalive support

[findleyr]

Add support for automatic keepalive 'ping' requests.

Design: https://github.com/modelcontextprotocol/go-sdk/blob/main/design/design.md#ping--keepalive

[rwjblue-glean]

👋 sorry for the late ping, I realize that I was supposed to confirm that it was ok for new contributors first (I didn't notice that part in the CONTRIBUTING.md at first 🤦 ).

I sent #39 to implement this.

[findleyr]

No worries, and thanks for reading CONTRIBUTING.md!

No harm done: we hadn't started working on this and your contribution is very welcome!