Flagging need for dynamic bucket names & regions

[DhenPadilla]

We're using Next-s3-upload to upload to a single bucket currently.
This sufficed for a single use case up until now as we have a strong security requirement to keep the current bucket that we're uploading to, to be completely READ-restricted from public access (i.e unaccessible to anything external to aws). This cannot change.

We want to continue using next-s3-upload for our new use case:
Uploading files to a new bucket which would have public read access.
Currently, next-s3-upload does not allow for dynamically allocating a bucket name and only saves to the S3_UPLOAD_BUCKET env var. This is fine for our initial use case, but definitely not moving forward as we may need more than 2 buckets but especially more than 1.

Note: Although we're using 1 region, I imagine we (or other users) may use multiple regions now in the future.

Opened PR: #169 to solve this too.

Opening this as an issue to create a thread for discussion if there are security risks and issues you may want to flag here.