chore: add tokio_unstable to known cfgs to avoid linter warns

Author: jlizen

lambda-runtime/Cargo.toml

@@ -14,6 +14,9 @@ categories = ["web-programming::http-server"]
 keywords = ["AWS", "Lambda", "API"]
 readme = "../README.md"
 
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tokio_unstable)'] }
+
 [features]
 default = ["tracing"]
 tracing = ["lambda_runtime_api_client/tracing"] # enables access to the Tracing utilities