Consider If-Modified-Since header

[Petitoto]

Subject of the issue

When OIDC clients try to get a user profile picture, they often use a If-Modified-Since flag to reduce the network usage. We should support this

Your environment

• Version of Hyperion: latest main
• Operating system: debian

Steps to reproduce

Run Hyperion and add an OIDC provider which triggers the profile picture and use a If-Modified-Since (eg: Nextcloud).

Expected behaviour

The profile picture should be downloaded only when it has been changed since the last download of the OIDC client.

Actual behaviour

The picture is always returned to the OIDC client