Allow to freeze/lock/hold/pin on a package version

It can be useful to prevent from any change (upgrade or downgrade) on a given package.
Add an option to nuxeoctl to pin/unpin on a given version.
In the Admin Center:

• gray uninstall button,
• gray upgrade buttons on other versions than the locked one (if pinned on version),
• add a "pin/unpin" button if...
  • locked?
  • system package?
  • ...