feat: Add KeepAlive

[jkroll-deepgram]

Support the KeepAlive feature to keep a websocket open for an additional 12 seconds without sending additional streaming audio data.

Basic usage:

deepgram = Deepgram(DEEPGRAM_API_KEY)
deepgramLive = await deepgram.transcription.live(PARAMS)
deepgramLive.keep_alive()

[jjmaldonis]

Looks good to me!

At some point it might be worth adding functionality to send this keep-alive automatically until the user manually closes the websocket connection. However, that would require an additional task/thread running at all times for each websocket to keep track of when the last message was sent to the websocket.