register a repository package server as a device's update source
usage: fx add-update-source [--repo-name NAME] [--persist] Configure target device to use a new update source. --repo-name NAME Name of the repository package server to use as the update source. --persist Enable persistence of repo metadata. Disabled by default. NAME: If no name is supplied, the name defaults to configured default repository. Note: This is a wrapper for `ffx target repository register`