[S3 Input] Add support for only iterating/accessing only specific folders or datapaths

[P1llus]

Certain hosted S3 buckets, managed by third parties, will give access to only specific datapaths, meaning that the S3 input will always get access denied, because it tries to iterate over the whole S3 bucket.

For example Cisco managed S3 buckets for Umbrella:
https://docs.umbrella.com/deployment-umbrella/docs/cisco-managed-s3-bucket

This only allows you to run the list-objects API inside the specified data path, so it would be great if we had an argument that defines the root datapath in which we run the list-objects API on, else it will just return 403 Access Denied.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)