[blob-service] Make S3 bucket configurable

Summary: Part of ENG-4573. Added a new --s3-bucket-name CLI option and BLOB_S3_BUCKET_NAME environment variable that allows to override the default commapp-blob S3 bucket. We need this to be able to use a separate bucket in staging environment.

Test Plan: Ran integration tests locally - confirmed that default S3 bucket is used. Ran integration tests with --s3-bucket-name set to non-existent bucket - they fail with NoSuchBucketError as expected. Repeated with the environment variable set instead.

Reviewers: jon, varun, michal

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8742