Add Google Gemini TTS API support

[shoryasethia]

Description

This PR adds support for Google Gemini's Text-to-Speech (TTS) API to Verbi, allowing users to use Google's latest TTS capabilities alongside existing providers.

Changes Made

• Added Google Gemini TTS integration to voice_assistant/text_to_speech.py
• Updated configuration to support gemini as a TTS model option
• Added API key management for Google API
• Updated requirements to include google-generativeai
• Updated documentation in README

Usage

Set TTS_MODEL = 'gemini' in config.py and add GOOGLE_API_KEY to your .env file.

References

• Google Gemini Speech Generation Documentation: https://ai.google.dev/gemini-api/docs/speech-generation

[shoryasethia]

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