Add ability to set realtime model

[mandryllo]

This PR:

• fixes hardcoded model selection when used in nodejs
• adds ability to set model when instantiating a new client
• adds ability to set model when connecting to a client (will override the one set when instantiating a client)

It didn't add ability to set model when using updateSession because per documentation model shouldn't be changed in session.update event.

Example:

const client = new RealtimeClient({ model: 'gpt-4o-realtime-preview' });
...
client.connect({ model: 'gpt-4o-mini-realtime-preview' });

[devpras22]

wow finally it works! awesome @mandryllo you made my day!