File Source: add Box storage provider

## Tell us about the problem you're trying to solve
Currently, the file source connector supports file providers such as GCS and S3.

[Box](https://www.box.com/en-ca/home) is also a cloud storage service where users can store/share files (similar to Google Drive)

## Describe the solution you’d like
Allow users to download/sync box files from a given Box file URL.

## Describe the alternatives you’ve considered or used
- Considered creating a new source for Box , however much of the logic from the file source would be shared.
- Adding a new provider to the file source for Box

## Are you willing to submit a PR?

Yes (in progress)