Added

• Added core entry to nrfutil search to indicate the current version of nRF Util itself and also print out a message beneath the search results if there is a new version of nRF Util to upgrade to with self-upgrade.
• Added a check for nrfutil self-upgrade when doing install or upgrade. A self-upgrade recommendation message will be printed after the install or upgrade output if a newer version of core is found on the package registry.