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/lib.rs

@@ -45,9 +45,13 @@ mod api;
 mod ir;
 mod types;
 
+pub mod utils;
+
 pub use self::{
     api::{
-        generate_runtime_api,
+        generate_runtime_api_from_bytes,
+        generate_runtime_api_from_path,
+        generate_runtime_api_from_url,
         RuntimeGenerator,
     },
     types::{