feat: Add string value prompt to SynthesisInput (#30942)

* feat: Add string value prompt to SynthesisInput

PiperOrigin-RevId: 802688528

Source-Link: googleapis/googleapis@acdae9a

Source-Link: googleapis/googleapis-gen@2118e2f
Copy-Tag: eyJwIjoiZ29vZ2xlLWNsb3VkLXRleHRfdG9fc3BlZWNoLXYxYmV0YTEvLk93bEJvdC55YW1sIiwiaCI6IjIxMThlMmYxNDAzYzE0N2YzOWJkMmI5YTIxZGU2NzMwODM5M2E3ZDcifQ==

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

google-cloud-text_to_speech-v1beta1/lib/google/cloud/texttospeech/v1beta1/cloud_tts_pb.rb

@@ -222,6 +222,12 @@ class Turn
         #     multi-speaker synthesis.
         #
         #     Note: The following fields are mutually exclusive: `multi_speaker_markup`, `text`, `markup`, `ssml`. If a field in that set is populated, all other fields in the set will automatically be cleared.
+        # @!attribute [rw] prompt
+        #   @return [::String]
+        #     This system instruction is supported only for controllable/promptable voice
+        #     models. If this system instruction is used, we pass the unedited text to
+        #     Gemini-TTS. Otherwise, a default system instruction is used. AI Studio
+        #     calls this system instruction, Style Instructions.
         # @!attribute [rw] custom_pronunciations
         #   @return [::Google::Cloud::TextToSpeech::V1beta1::CustomPronunciations]
         #     Optional. The pronunciation customizations are applied to the input. If