Add Unified Search Provider for LibreSign

[vitormattos]

Is your feature request related to a problem? Please describe.

Currently, LibreSign does not appear in the results of Nextcloud's "Unified Search". It would be useful to allow LibreSign files to be found through the platform's unified search.

Describe the solution you'd like

Implement a Search Provider for LibreSign, following the official Nextcloud documentation:
https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/search.html

The provider should be registered in the main application class (Application) and return files managed by LibreSign, making them discoverable via Unified Search.

A good suggestion for the LibreSign search provider would be:

• id: libresign
• name: LibreSign

These follow the convention used by other Nextcloud apps and are clear, concise, and unique.

Describe alternatives you've considered

• Keep the current behavior, without unified search integration.
• Use manual searches within LibreSign.

Additional context

• If you are unsure how to structure the template, start by looking at the example template and adapt it for LibreSign.
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