Allow setting visibility of status

[joschi]

What’s missing?

The visibility of status updates is currently hard-coded to unlisted (to avoid spamming the local timeline).

There should be a setting to override the default visibility of status updates posted by this GitHub action or even allow setting it per-toot.

Why?

It's often desirable to at least set the default visibility level for status updates in order to show toots on the public timeline or only to a specific group.