feat(genai): add text generation samples (2)

[jdomingr]

Add new samples for text generation:

• Text generation async with text.
• Text generation with content configs.
• Text generation with Gcs Audio.
• Transcript with Gcs Audio.

All local tests have passed

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• genai/api/GenAI.Samples/TextGeneration/TextGenAsyncWithTxt.cs:17, tag googlegenaisdk_textgen_async_with_txt
• genai/api/GenAI.Samples/TextGeneration/TextGenConfigWithTxt.cs:17, tag googlegenaisdk_textgen_config_with_txt
• genai/api/GenAI.Samples/TextGeneration/TextGenTranscriptWithGcsAudio.cs:17, tag googlegenaisdk_textgen_transcript_with_gcs_audio
• genai/api/GenAI.Samples/TextGeneration/TextGenWithGcsAudio.cs:17, tag googlegenaisdk_textgen_with_gcs_audio

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[yyyu-google]

look good to me