Provide mechanism to override libraries with custom/patched versions

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Is your feature request related to a problem? Please describe.
A lot of times, system admins have patches or modifications to libraries that they need to apply for their specific use cases. Many times, they have submitted a PR, it is yet to be approved or has gone stale, but they still need the feature. However, directly editing core files is a bad idea and causes system integrity checks to fail.

Describe the solution you'd like
It would be great if there was a way to add library overrides to the system so that the autoloader picks them up at runtime. Perhaps they could be placed in config/overrides/[path-of-lib-from-root], for example config/overrides/lib/private/DB/AdapterMySQL.php)

Describe alternatives you've considered
I did write a very hacky piece of logic in a *.config file that implements this functionality, but it's not ideal, and it would be good if there was something officially supported.

Additional context
An example, this PR has been open forever, and there are other users who rely on this functionality:
https://github.com/nextcloud/user_saml/pull/697

Until the PR is merged, it would be nice to be able to override the default libraries to provide the desired features.

Start by examining the autoloader and the example path config/overrides/lib/private/DB/AdapterMySQL.php mentioned in the issue. Review the linked nextcloud/user_saml#697 pull request for the motivating use case. Done would mean a documented, officially supported override mechanism that preserves system integrity checks.