Google STT plugin

[roertbb]

Following Google STT implementation has been heavily inspired by:

• Python Google STT implementation - https://github.com/livekit/agents/tree/main/livekit-plugins/livekit-plugins-google
• Node.js Deepgram STT implementation - https://github.com/livekit/agents-js/blob/main/plugins/deepgram/src/stt.ts

The Google SpeechClient v2 SDK for Node.js has been poorly documented and I've been following some trial and error approach (see _streamingRecognize instead of streamingRecognize, shape of initial streaming recognize config request) in which I ended up with the following working implementation.

Also, during the tests I've noticed that Google closes the connection with timeout error after it does not receive audio data. It can be observed during longer agent responses during which the audio is not transformed. To alleviate the problem I've implemented sending empty audio frames periodically, when user does not speak - see keepalive interval implementation. It was inspired on Deepgram implementation.

If there is some additional work required to integrate new plugin I'd be grateful for guidance as I'm first time contributor here.

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