Add per-filesystem S3-compatible endpoint support

[timkelty]

Summary

• add a per-filesystem endpoint setting so MinIO and other S3-compatible services can be configured without relying on a global env var
• keep the existing Amazon S3 behavior when no endpoint is configured, including the current STS-backed credential flow
• update bucket loading and settings docs so custom endpoints are used directly and no longer force AWS-style URL assumptions

Verification

• composer phpstan
• composer check-cs