Perf: generate and ship authoritative Composer class loader

[ChristophWurst]

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

This app does not ship its own classloader, so the non-optimized fallback loader is used.

Describe the solution you'd like

https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/classloader.html#replacing-nextcloud-s-autoloader

Describe alternatives you've considered

N/a

Additional context

composer.json has already been configured for an optimized class loader, but

• The vendor/autoload.php vendor/composer/* files are missing in the release
• There is no PSR-4 mapping for this app's own class files

[miaulalala]

#1544