add support for speechify service

[sourabhsinha396]

Add Speechify TTS Service Support

This PR adds support for Speechify's text-to-speech API as a new service in manim-voiceover.

Features

• Implements a new SpeechifyService class that interfaces with the Speechify API
• Includes token-based authentication with automatic refresh when tokens expire
• Properly handles caching to avoid unnecessary API calls
• Supports all standard manim-voiceover functionality including global speed adjustment

Good things

• Uses token-based authentication for better security (obtains access tokens using the API key)
• Implements okayish error handling and logging
• Gracefully recovers from 401 unauthorized errors by automatically refreshing the token

Usage Example

from manim_voiceover.services.speechify import SpeechifyService

# Initialize the service with your Speechify voice ID
speechify = SpeechifyService(
    voice_id="your_voice_id_here",
    model="simba-english",  # optional
    api_key="your_api_key_here"  # or set via SPEECHIFY_API_KEY environment variable
)

# Use it in your scene
class MyScene(VoiceoverScene):
    def construct(self):
        self.init_voiceover(speechify)
        with self.voiceover("Hello, this is a test using Speechify."):
            self.play(...)

[sourabhsinha396]

ModernQuizWithVoiceover.mp4

Sample output, I could get this from speechify.