Use S3 prefix when storing intermediate files

[moreyhat]

Is your feature request related to a problem? Please describe.
VFL client is able to access the intermediate files stored in the S3 bucket such as embeddings and gradients of other VFL clients.

Describe the solution you'd like
Introduce S3 prefix for intermediate files and corresponding IAM policy for each client to restrict the access to the intermediate files from other VFL clients

Describe alternatives you've considered
N/A

Additional context
N/A