Clear cache after the vault has been locked

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/ios/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/ios/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Add an option so that a vault can clear its cache after it has been locked

### Motivation

Even though the local device is not Cryptomator's security target, some users would feel more comfortable if there is as little decrypted content as possible on the device (even though it's in a sandbox).

Adding a "Clear Cache After Lock" option (per vault) would make sure that decrypted content is automatically removed without having to think about it. However, it shouldn't be enabled by default because it would cause usability drawbacks (often used files have to be re-downloaded, thumbnails get lost).

### Considered Alternatives

A user can already manually clear the cache to achieve the same goal.

### Anything else?

_No response_