Allow the use of 'prefix' in S3 buckets

[mbartsch]

• Resolves: #

Summary

Adds an optional Object key prefix parameter to the S3-Compatible Object Storage external storage backend.

This allows scoping a mount to a sub-path within a shared bucket rather than requiring exclusive use of the entire bucket. For example, setting the
prefix to nextcloud/$user/ will store each user's files under their own namespace within the bucket, with $user being substituted automatically at
mount time.

What changed:

• Backend/AmazonS3.php: new optional prefix field exposed in the admin UI
• Storage/AmazonS3.php: prefix is parsed and normalized in the constructor (leading slashes stripped, trailing slash enforced), then applied
  transparently to all S3 key operations (read, write, delete, list, copy, presigned URLs). Keys returned from S3 listing responses have the prefix
  stripped before being stored in the in-memory cache or yielded as directory entries. The storage ID includes the prefix so two mounts to the same
  bucket with different prefixes are treated as distinct storages.

Variable substitution: $user (and any other registered placeholder) works in the prefix field automatically, since all backend options pass through
the existing applyConfigHandlers() pipeline before the storage is constructed.

TODO

• Add unit tests for addPrefix / stripPrefix helpers and constructor normalization
• Add integration test covering mount with a prefix set

Checklist

• Code is properly formatted (https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/continuous_integration.html#linting)
• Sign-off message (https://github.com/src-d/guide/blob/master/developer-community/fix-DCO.md) is added to all commits
• Tests (unit (https://docs.nextcloud.com/server/latest/developer_manual/app_development/tutorial.html#unit-tests), integration
  (https://docs.nextcloud.com/server/latest/developer_manual/app_development/tutorial.html#integration-tests), api and/or acceptance) are included
• Screenshots before/after for front-end changes (admin external storage form shows new optional field)
• Documentation (manuals (https://github.com/nextcloud/documentation/) or wiki) has been updated or is not required
• Backports requested (https://github.com/nextcloud/backportbot/#usage) where applicable
• Labels added (https://github.com/nextcloud/server/labels) — enhancement, 3. to review, files_external
• Milestone added (https://github.com/nextcloud/server/milestones) for target branch/version

AI (if applicable)

• The content of this PR was partly or fully generated using AI