Add runtime_metadata_url to pull metadata directly from a node (#689)

* Allow metadata to be pulled directly from a node with runtime_metadata_url

* Update docs

* https too, and abstract out block_on fn

* tweak a comment in the example

examples/examples/custom_metadata_url.rs

@@ -0,0 +1,12 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+// If you'd like to use metadata directly from a running node, you
+// can provide a URL to that node here. HTTP or WebSocket URLs can be
+// provided. Note that if the metadata cannot be retrieved from this
+// node URL at compile time, compilation will fail.
+#[subxt::subxt(runtime_metadata_url = "wss://rpc.polkadot.io:443")]
+pub mod polkadot {}
+
+fn main() {}