Add option to figure HTTP2 keep-alive pings to `HTTP2Settings`

[edsko]

The http2 library does not yet support sending keep-alive pings; this will need to be addressed first.

See https://grpc.io/docs/guides/keepalive/ for a discussion from a gPRC perspective.
Apparently it is also important for performance: see https://grpc.io/docs/guides/performance/ .