feat: add conversation type support for Twilio SMS chat transport

This update adds conversation type support to TwilioSmsChatTransport and introduces a dial timeout option for TransferPlan. These enhancements provide better control over chat conversation types and transfer call behavior.

Key changes:
- Add TwilioSmsChatTransportConversationType enum with "chat" literal type
- Add conversation_type field to TwilioSmsChatTransport to specify call type
- Add dial_timeout field to TransferPlan for SIP DIAL operation timeout control
- Update User-Agent header to use version placeholder instead of hardcoded value
- Export new conversation type in module __all__ lists

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