Add --with-settings option for the "load" command

[vittorius]

Is your feature request related to a problem? Please describe.
Cannot load settings using GitHub token and Gist ID that are already present in editor's settings. It's convenient to do so in case of syncing config between 2 Zed instances where the extension is installed.

Describe the solution you'd like

• Have a value-less option --with-settings for the zed-settings-sync-cli load that will use creds from the "settings-sync" entry in the Zed config file. If this entry is missing or invalid, exit with an error.
• Print an info message when using creds from the settings file.