Whitelist cfg attributes

As we did in rust-bitcoin/rust-bitcoin#2785.

rust-lang/rust#124800 has been fixed and we can update our nightly
version by whitelisting all cfgs that are used.

Cargo.toml

@@ -39,3 +39,6 @@ socks = { version = "0.3.4", optional = true}
 
 [workspace]
 members = ["fuzz", "integration_test"]
+
+[lints.rust]
+unexpected_cfgs = { level = "deny", check-cfg = ['cfg(jsonrpc_fuzz)'] }