Add c.Heartbeat

[nhooyr]

See #265 (comment)

[Rambatino]

Hi @nhooyr! I take it you're on sabbatical, but is there any movement on this?

[jiangjiancc]

@nhooyr I want to know how this function is progressing？

[benma]

Bump - this seems important to include for a good out of the box experience. I just waded through a lot of comments in the issue tracker to figure out if ping/pongs are needed and how to add them.