Add a destination to an existing app's workflow from a configuration file

Accepts the same YAML schema as createAppFromConfig, but instead of creating a new app, adds a new destination to the given app's existing workflow. The YAML's name/icon become the destination's name and icon; integrations are installed as new app integrations from existing org providers; appSettings.branchSettings are stored as a per-destination branch config; stepGroups are cloned into the workflow as new step configs. Other fields (appGroup, appUsers, appStoreReleaseSettings, buildDistro, tagSettings, featureAffiliations) are ignored.