STT EVAL: adding get endpoints and audio files signed url

[nishika26]

Describe the current behavior
Currently there is only one endpoint in audio files module, which is just to upload it to s3 and create an entry in the files table. Now that we are buliding an UI for STT Eval, we feel the need to have get endpoints. we require signed url to be present in the response body of those endpoints as well since we want to build the support for playing uploaded audios in the UI.

Describe the enhancement you'd like
We need to build get endpoints for audio files module and also give back signed url in the response bodies of those get endpoints so that users can listen to what they uploaded.