Streamlit-audiorecorder

Add the ability to style the buttons with CSS.

Add the possibility to use the [react-audio-recorder](https://github.com/samhirtarif/react-audio-recorder) visualizer instead of buttons.

Add pausing capability by adding a pause button if the `pause_prompt` argument is specified.

This is a breaking change release.

The package has been updated to work with the newer version of streamlit, and:
- It now returns a pydub AudioSegment, which requires the installation of ffmpeg and pydub.
If I get feedback that it was useful to just record audio without the need for ffmpeg, I can consider adding a flag to return only the byte code.
- It follows streamlit theme, as explained [here](https://docs.streamlit.io/library/components/components-api#working-with-themes).
In particular, we use the theme variables: `textColor`, `backgroundColor`, `secondaryBackgroundColor`.

Note:
The audio is recorded by default in webm format in Firefox and Chrome, but Safari does not handle it and (likely) returns mp4.
I chose not to specify the format both in the frontend and the backend, given that ffmpeg handles it properly in all my tests, and it makes things simpler.