feat: add local caching for remote IdP logos

[Menrath]

Summary

• Store IdP logos/icons in a local plugin filearea (idplogo), with itemid matching the auth_saml2_idps.id.
• Generate deterministic filenames by hashing the source URL (preserving extension).
• Provide helper methods to cache, retrieve, and delete cached logos.

Why

• Avoid loading third‑party resources at runtime and prevent outbound requests on page render.
• Improve privacy and compliance: serving assets locally helps meet GDPR expectations by avoiding unsolicited requests to external hosts and potential user tracking.

Tests

Are still missing. ToDo.

Notes

• No database schema changes.
• Existing IdPs without a logo continue to work.
• Remote network access only occurs during upgrade or metadata refresh tasks, never during normal page rendering.

[Menrath]

I consider this PR to be ready for review. I offer to write tests, after a first round of feedback.