E2E testing: Fetch runtime metadata automatically

[cmichi]

Follow-up to #1395.

The current E2E testing framework still contains a hardcoded

#[subxt::subxt(runtime_metadata_path = "metadata/contracts-node.scale")]
pub(super) mod api {}

Instead of requiring this test fixture in the ink repository (exported via subxt metadata > contracts-node.scale while the node is running), we should fetch it automatically, getting rid of requiring users to do this step.

With paritytech/subxt@d03e599, subxt got the possibility of supplying a

#[subxt::subxt(runtime_metadata_url = "wss://rpc.polkadot.io:443")]

We already support #[ink_e2e::test(ws_url = "…")], so ideally we would use that supplied URL for the runtime_metadata_url.