feat(http2): add HTTP2 keep-alive support for client and server

This adds HTTP2 keep-alive support to client and server connections
based losely on GRPC keep-alive. When enabled, after no data has been
received for some configured interval, an HTTP2 PING frame is sent. If
the PING is not acknowledged with a configured timeout, the connection
is closed.

Clients have an additional option to enable keep-alive while the
connection is otherwise idle. When disabled, keep-alive PINGs are only
used while there are open request/response streams. If enabled, PINGs
are sent even when there are no active streams.

For now, since these features use `tokio::time::Delay`, the `runtime`
cargo feature is required to use them.

src/client/mod.rs

@@ -933,6 +933,7 @@ impl Builder {
         self.pool_config.max_idle_per_host = max_idle;
         self
     }
+
     // HTTP/1 options
 
     /// Set whether HTTP/1 connections should try to use vectored writes,
@@ -1036,6 +1037,59 @@ impl Builder {
         self
     }
 
+    /// Sets an interval for HTTP2 Ping frames should be sent to keep a
+    /// connection alive.
+    ///
+    /// Pass `None` to disable HTTP2 keep-alive.
+    ///
+    /// Default is currently disabled.
+    ///
+    /// # Cargo Feature
+    ///
+    /// Requires the `runtime` cargo feature to be enabled.
+    #[cfg(feature = "runtime")]
+    pub fn http2_keep_alive_interval(
+        &mut self,
+        interval: impl Into<Option<Duration>>,
+    ) -> &mut Self {
+        self.conn_builder.http2_keep_alive_interval(interval);
+        self
+    }
+
+    /// Sets a timeout for receiving an acknowledgement of the keep-alive ping.
+    ///
+    /// If the ping is not acknowledged within the timeout, the connection will
+    /// be closed. Does nothing if `http2_keep_alive_interval` is disabled.
+    ///
+    /// Default is 20 seconds.
+    ///
+    /// # Cargo Feature
+    ///
+    /// Requires the `runtime` cargo feature to be enabled.
+    #[cfg(feature = "runtime")]
+    pub fn http2_keep_alive_timeout(&mut self, timeout: Duration) -> &mut Self {
+        self.conn_builder.http2_keep_alive_timeout(timeout);
+        self
+    }
+
+    /// Sets whether HTTP2 keep-alive should apply while the connection is idle.
+    ///
+    /// If disabled, keep-alive pings are only sent while there are open
+    /// request/responses streams. If enabled, pings are also sent when no
+    /// streams are active. Does nothing if `http2_keep_alive_interval` is
+    /// disabled.
+    ///
+    /// Default is `false`.
+    ///
+    /// # Cargo Feature
+    ///
+    /// Requires the `runtime` cargo feature to be enabled.
+    #[cfg(feature = "runtime")]
+    pub fn http2_keep_alive_while_idle(&mut self, enabled: bool) -> &mut Self {
+        self.conn_builder.http2_keep_alive_while_idle(enabled);
+        self
+    }
+
     /// Set whether to retry requests that get disrupted before ever starting
     /// to write.
     ///