[6.x] Add file permission config option for the File cache store

[klimov-paul]

Adds 'permission' configuration option for the File cache store.

At the present state file cache writes files with the default umask, which is usually allows writing (deleting) only for the owner.

This creates trouble in some environments, when cache is written by web-server (e.g. nginx) and then accessed from the artisan console.

In particular this make php artisan cache:clear command to silently fail to remove file cache created by web-server.

This PR adds 'permission' config option for the FileStore, in the same manner as for Stream Logger.