[Finder] adds a way to ignore AccessDeniedException

This PR adds a `Finder::ignoreUnreadableDirs()` method which tells `Finder` to ignore unreadable directories instead of throwing an `AccessDeniedException`.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6981