Publicize: add option to set privacy when sharing to Mastodon

### Impacted plugin

Social

### What

I would like to be able to set the Publishing Level for Mastodon posts.

### How

Based on observed behaviour, it appears that a Mastodon post created by Jetpack uses the default Publishing Level for my Mastodon account.

I would like to be able to select one of Public, Unlisted, or Followers Only. This could be a plugin setting that is always used, or could be set in the sharing panel in the post editor sidebar, or both.

Start by locating the Social plugin's Mastodon sharing integration and the post editor sharing settings. Check how Jetpack currently selects the Mastodon publishing level and how the sharing request is built. Done means users can choose Public, Unlisted, or Followers Only through a documented setting or sharing control, with tests covering the selected value.

Written by the indexing model from the issue text.