TelemetryEndpoints must be valid MutliAddr URL (#5069)

* Check for url validity when creating TelemetryEndpoints

* Update code that used TelemetryEndpoints::new()

* Update commennts that referred to TelemetryEndpoints::new()

* Add tests for telemetry

* Fix typo and fix code in docs

* Return error on failing to override telemetry

* Use expect instead of suppressing errors on must-be-valid telemetry endpoints

* Update telemetry unit tests to use expect instead of unwrap

* Implement custom deserializer for TelemetryEndpoints

* Fix typo

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/bin/node/cli/src/chain_spec.rs

@@ -159,7 +159,8 @@ pub fn staging_testnet_config() -> ChainSpec {
 		"staging_testnet",
 		staging_testnet_config_genesis,
 		boot_nodes,
-		Some(TelemetryEndpoints::new(vec![(STAGING_TELEMETRY_URL.to_string(), 0)])),
+		Some(TelemetryEndpoints::new(vec![(STAGING_TELEMETRY_URL.to_string(), 0)])
+			.expect("Staging telemetry url is valid; qed")),
 		None,
 		None,
 		Default::default(),