Make the `--mounter` option optional. Use defaults (as with the desktop app) when it is not present

### Please agree to the following

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

### Summary

I do not want to provide the `--mounter' option all the time. We should use the same strategy here as for the desktop application, provide defaults and if the user wants to use something else, use the value of that option.

### What software is involved?

- Operating System: Not relevant
- Cryptomator CLI: 0.6.0

### Volume Type

None

### Steps to Reproduce

1. `echo "foo" | cryptomator-cli unlock /home/foo/vault/ --mountPoint=/home/foo/mounted --password:stdin`

### Expected Behavior

Vault gets unlocked using the default mount provider

### Actual Behavior

`Error: Missing required argument(s): --mounter=fully.qualified.ClassName` is shown

### Reproducibility

Always

### Relevant Log Output

_No response_

### Anything else?

_No response_

Beginne mit dem CLI-Entsperrablauf und der Verarbeitung der Option `--mounter`, und vergleiche ihn anschließend mit der standardmäßigen Mount-Strategie der Desktopanwendung. Überprüfe den gemeldeten Befehl ohne `--mounter` und bestätige, dass er den Standard-Provider verwendet, während eine explizit angegebene Option weiterhin berücksichtigt wird.

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