Added several parameter checks and test cases for parameter checks

* Added parameter checks to `NetClientImpl`, `CaOptionsImpl`, `FileSystemImpl`, `HttpClientImpl`, `HttpClientRequestImpl`, `SocketAddressImpl`, `WebSocketConnectOptionsImpl`.
* Added helper class `Arguments` for argument validation analogous to `Objects.requireNonNull()`. It's used only in few classes like `WebSocketConnectOptionsImpl` for review.
* Refactored parameter checks to use consistently `Objects.requireNonNull()`.
* Added unit tests for several existing parameter checks.
* Fixed small typo in `CONTRIBUTING.md`.