Config for web3signer keep-alive (#5007)

* Allow tweaking connection pool settings

* Build docker image

* Fix imports

* Merge tag 'v4.6.0' into web3signer-keep-alive

v4.6.0

* Delete temp docker build stuff

* Fix tests

* Merge remote-tracking branch 'origin/unstable' into web3signer-keep-alive

* Update CLI text

validator_client/src/cli.rs

@@ -367,4 +367,24 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     constructed by builders, regardless of payload value.")
                 .takes_value(false),
         )
+        /*
+         * Experimental/development options.
+         */
+        .arg(
+            Arg::with_name("web3-signer-keep-alive-timeout")
+                .long("web3-signer-keep-alive-timeout")
+                .value_name("MILLIS")
+                .default_value("90000")
+                .help("Keep-alive timeout for each web3signer connection. Set to 'null' to never \
+                       timeout")
+                .takes_value(true),
+        )
+        .arg(
+            Arg::with_name("web3-signer-max-idle-connections")
+                .long("web3-signer-max-idle-connections")
+                .value_name("COUNT")
+                .help("Maximum number of idle connections to maintain per web3signer host. Default \
+                       is unlimited.")
+                .takes_value(true),
+        )
 }