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

codegen/src/utils/mod.rs

@@ -0,0 +1,12 @@
+mod fetch_metadata;
+
+// easy access to this type needed for fetching metadata:
+pub use jsonrpsee::client_transport::ws::Uri;
+
+pub use fetch_metadata::{
+    fetch_metadata_bytes,
+    fetch_metadata_bytes_blocking,
+    fetch_metadata_hex,
+    fetch_metadata_hex_blocking,
+    FetchMetadataError,
+};