feat: enable non-blocking transcription saving

[chc-tw]

This pull request introduces asynchronous functionality to the transcribe_demo.py script, allowing for non-blocking operations and periodic saving of transcriptions. The most important changes include adding a coroutine for saving transcripts, modifying the main function to use asyncio, and updating the recording callback to accommodate asynchronous behavior.

Asynchronous Functionality:

• Added save_transcript coroutine to save the transcript to a file periodically.
• Modified the main function to use asyncio.run for running the asynchronous main function.

Command Line Arguments:

• Added --output_file argument to specify the file to save the transcript to.

Recording Callback Updates:

• Started the save_transcript coroutine within the record_callback function using asyncio.create_task.
• Replaced sleep calls with await asyncio.sleep to allow other coroutines to run and prevent blocking.
• Ensured the save task is canceled and awaited before exiting the program.