Support transcriptions and update to latest API changes

[jblwilliams]

First of all, thank you for making this library! I was testing this with Realtime Transcriptions and came across several issues that I've attempted to patch.

I've made several changes as I was testing but some are breaking changes. I wanted to open this PR to invite further discussion on what the best way to evolve the API would be.

Major Changes

• Remove model from connect API and rely on configuration changes or initial client_secrets API call to configure
• Add conversationItemAdded server event
• Use previousItemId to place entries in correct position

I needed to remove Model? from the conversation.connect call because it was overriding the initial session object I created server side to create an ephemeral key. I'm doing transcription not realtime and the override was forcing an invalid change. I could see an argument either way for only supporting session updates after the connection is established. The OpenAI API docs seem to encourage this pattern via example here since they're configuring the session to get the client_secret/ephemeral_key

[Ghussy]

I appreciate you working in this 🙏🏽 This was a blocker for me