Keep HTTP/2 connections alive with ping frames

[timostamm]

Is your feature request related to a problem? Please describe.
Especially with long-running streams that idle most of the time, it is possible that a connection dies without the client noticing.
If this happens, the Node.js client simply hangs.

Describe the solution you'd like
Send PING frames in an interval to monitor the connection state in all Transports from @bufbuild/connect-node with httpVersion: "2". Raise an error if the peer does not respond within a certain duration.

Additional context
gRCP Core support it, as described in https://github.com/grpc/grpc/blob/master/doc/keepalive.md

[fubhy]

I was looking for this just now. Perfect timing :-)