feat: implement and test RCS channel support

[kojiromike]

Add RCS as a messaging channel in the Sinch Conversations module.

The Conversation API app (SmiTTY Talk) already lists RCS as an available channel. This involves:

• Setting up an RCS Agent in the Sinch dashboard (requires Google verification)
• Associating the RCS agent with the SMS phone number for fallback
• Enabling the RCS channel in the Conversation API app
• Testing RCS message delivery with fallback to SMS