feat: Add IFilenameValidator to have one consistent place for filename validation

[susnux]

• Part of 🪟📁📄 Ability to enforce windows compatible file names on the server #44963

Summary

This is the first part, split #45151 to make review easier.

This adds a new interface IFilenameValidator that allows validating filenames to have one consistent location for filename handling.
We need two¹ functions:

1. Validation with meaningful messages (as errors)
2. Validation for if with boolean return

¹ We need in some rare private filesystem locations a third function: We need to check if a file is forbidden but can not validate all other restrictions, because we use our filesystem API also for setup where we use possibly forbidden characters as we are only setup Nextcloud itself not files later synced.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

Follow up with usage here: #46375