Add support for proxying in `WebsocketClient`

This patch was authored by @rnro and released by @0xTim.

Added support for TLS and plain text proxying of Websocket traffic.

• In the TLS case a CONNECT header is first sent establishing the proxied traffic.

• In the plain text case the modified URI in the initial upgrade request header indicates to the proxy server that the traffic is to be proxied.

• Use NIOWebSocketFrameAggregator to handle aggregating frame fragments. This brings with it more protections e.g. against memory exhaustion.

• Accompanying config has been added to support this change.

This change also includes some performance gains by reducing the allocation and copies necessary to send ByteBuffer and ByteBufferView through WebSocket.send.

• Sending ByteBuffer or ByteBufferView doesn’t require any allocation or copy of the data. Sending a String now correctly pre allocates the ByteBuffer if multibyte characters are present in the String.

• Remove custom random websocket mask generation which would only generate bytes between UInt8.min..<UInt8.max, therefore excluding UInt8.max i.e. 255.