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

testing/ui-tests/src/incorrect/need_url_or_path.rs

@@ -0,0 +1,4 @@
+#[subxt::subxt()]
+pub mod node_runtime {}
+
+fn main() {}

testing/ui-tests/src/incorrect/need_url_or_path.stderr

@@ -0,0 +1,7 @@
+error: One of 'runtime_metadata_path' or 'runtime_metadata_url' must be provided
+ --> src/incorrect/need_url_or_path.rs:1:1
+  |
+1 | #[subxt::subxt()]
+  | ^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `subxt::subxt` (in Nightly builds, run with -Z macro-backtrace for more info)

testing/ui-tests/src/incorrect/url_and_path_provided.rs

@@ -0,0 +1,7 @@
+#[subxt::subxt(
+    runtime_metadata_path = "../../../artifacts/polkadot_metadata.scale",
+    runtime_metadata_url = "wss://rpc.polkadot.io:443"
+)]
+pub mod node_runtime {}
+
+fn main() {}

testing/ui-tests/src/incorrect/url_and_path_provided.stderr

@@ -0,0 +1,10 @@
+error: Only one of 'runtime_metadata_path' or 'runtime_metadata_url' can be provided
+ --> src/incorrect/url_and_path_provided.rs:1:1
+  |
+1 | / #[subxt::subxt(
+2 | |     runtime_metadata_path = "../../../artifacts/polkadot_metadata.scale",
+3 | |     runtime_metadata_url = "wss://rpc.polkadot.io:443"
+4 | | )]
+  | |__^
+  |
+  = note: this error originates in the attribute macro `subxt::subxt` (in Nightly builds, run with -Z macro-backtrace for more info)