Add a checkbox to allow spotlight search in OSX GUI

### Summary

Add a checkbox to the OSX GUI to enable spotlight search for convince and easy of use.

### Motivation

It's a pain finding out that that Spotlight is not indexing those volumes after install and finding out how to enable it wasn't obvious. The check box could under the hood just amend to the custom mount commands "-oallow_root -olocal"

### Considered Alternatives

Adding better documentation or an FAQ to the app.

### Additional Context

Not sure if it can be added or be done but I'm gathering the actual encrypted files should be excluded in Spotlight's indexing via the Privacy tab, not sure if this is possible via the Cryptomator GUI but if so it would be good to add that has another step.

Start by locating the macOS GUI and the code that builds custom mount commands. Check how GUI options are represented and passed to mounting, then determine how the requested checkbox should control the additional options. Done should include a usable checkbox and confirmed Spotlight behavior, including whether encrypted files can be excluded from indexing.

Written by the indexing model from the issue text.