Enh: Allow sharing with federated users

[Ceelight]

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Describe the bug A clear and concise description of what the bug is.

We use trusted servers in the sharing part to synchronise the users of another instance. This works without any problems when sharing folders or files. The Forms app does not seem to use this - the users do not appear in the sharing selection menu.

Expected behavior A clear and concise description of what you expected to happen.

When entering a user from the other instance, this user should be displayed in the Share field and be selectable.

Nextcloud (please complete the following information):

• Nextcloud-Version: Nextcloud Hub 8(29.0.4)
• Forms-Version: 4.2.4

Desktop (please complete the following information):

• OS: MacOS
• Browser: Brave
• Version: Version 1.68.141 Chromium: 127.0.6533.120