fix(webtransport): add proper framing

WebTransport being a stream-based protocol, the chunking boundaries are
not necessarily preserved. That's why we need a header indicating the
type of the payload (plain text or binary) and its length.

We will use a format inspired by the WebSocket frame:

- first bit indicates whether the payload is binary
- the next 7 bits are either:
  - 125 or less: that's the length of the payload
  - 126: the next 2 bytes represent the length of the payload
  - 127: the next 8 bytes represent the length of the payload

Reference: https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API/Writing_WebSocket_servers#decoding_payload_length

Related:

- https://github.com/socketio/engine.io/issues/687
- https://github.com/socketio/engine.io/issues/688