Make VoicePipelineAgent generic

[martin-purplefish]

Now callers can say

MyVoicePipelineAgent = VoicePipelineAgent[MyAgentFunctionContext, openai.LLM, openai.TTS, silero.VAD, deepgram.STT]
...
def before_llm_cb(agent: MyVoicePipelineAgent, chat_ctx: llm.ChatContext):
   agent.fnc_ctx # is a MyAgentFunctionContext type

agent = MyVoicePipelineAgent(before_llm_cb=before_llm_cb)

[changeset-bot]

🦋 Changeset detected

Latest commit: 418143d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
livekit-agents	Patch

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[martin-purplefish]

PTAL @theomonnom @davidzhao