feat: toggle off plugin repo sync

[jthegedus]

Is your feature request related to a problem? Please describe.

The plugin repo sync ocurrs every 60 minutes and can be configured via plugin_repository_last_check_duration, however there is no way to turn off the sync completely.

Describe the solution you'd like

Allow plugin_repository_last_check_duration to be set to the special value of never and bypass the sync completely.

Describe similar asdf features and why they are not sufficient

Setting plugin_repository_last_check_duration to 999999999mins (1902 years) is an option, but not the same as completely turning off the sync.

Describe workarounds you've considered

NA

Additional context

This was raised in #957 (comment)