[encryption] store keys by fileId instead of path

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Is your feature request related to a problem? Please describe.
In Collectives, we can have different paths to a file by user (when a user changes their user_folder user setting). This leads to inaccessible files with server-side encryption as the encryption keys are stored by file path. The broken user experience is tracked in https://github.com/nextcloud/collectives/issues/2326 and https://github.com/nextcloud/collectives/issues/2284.

Describe the solution you'd like
In order to fix this, we should switch to fileId based key storage instead of file path based key storage.

Describe alternatives you've considered

• We could use the storage path instead for ISystemMountPoint paths, but this would come with its own problems. E.g. appdata is excluded from encryption and Collectives files live in appdata directory.
• We could introduce a way to map different user mountpoint paths to the same encryption key path.