Add ability to retain files from S3

[mingfang]

Use Case

Currently retainFiles() receive files and then save the files to S3 after retaining.

In our case our files are already in S3.
It would be more efficient to be able to retain directly from S3, e.g. retainFiles("s3 url").

Problem Statement

sending files in the retainFiles() call can be a problem when
a)files are too big
b)numerous files 10K+
c)result in double storage if the files originates from S3

How This Feature Would Help

efficiently retain files from S3

Proposed Solution

No response

Alternatives Considered

No response

Priority

Nice to have

Additional Context

No response

Checklist

• I would be willing to contribute this feature