Use URL library to allow custom query parameters when connecting to client

[jarrilla]

✅ All tests passed.

Current behavior hardcodes query URL formation, which prevents users from adding their own parameters.

Using URL library to parse incoming req.url will behave the same (still add model parameter ?model=gpt-4o....), but allow users to define extra parameters when connecting.

E.g. (using Vite)

const baseUrl = `${import.meta.env.VITE_API_BASE_URL}/stream`;
const url = new URL(baseUrl);
url.searchParams.append('sessionId', props.sessionId);

const client = new RealtimeClient({
  url: url.toString(),
  // url: "localhost:3000/stream?sessionId=1234" // this would also work
});