Add new `ui-dashboard-harvester-legacy-plugin` setting to support Harvester UI Plugin work - This supports step 3 from https://github.com/rancher/dashboard/issues/4109#issuecomment-1239387071 - The dashboard will load the harvester ui plugin contained in the harvester cluster, except when the cluster does not contain one (aka legacy clusters) - For those cases the dashboard will fall back on one the dashboard itself contains - However to support QA testing pre-release updates to this plugin... we supply an override referring to the `-head` build   - This is the same pattern as UI_INDEX / UI_DASHBOARD_INDEX and QA validation of pre-release features
Add new `ui-dashboard-harvester-legacy-plugin` setting to support Harvester UI Plugin work
- This supports step 3 from https://github.com/rancher/dashboard/issues/4109#issuecomment-1239387071
- The dashboard will load the harvester ui plugin contained in the harvester cluster, except when the cluster does not contain one (aka legacy clusters)
- For those cases the dashboard will fall back on one the dashboard itself contains
- However to support QA testing pre-release updates to this plugin... we supply an override referring to the `-head` build
  - This is the same pattern as UI_INDEX / UI_DASHBOARD_INDEX and QA validation of pre-release features

> Note - This is only used for legacy clusters. New clusters (that support ui plugins) will themselves provide the QA/-head build to test

rancher/rancher,rancher/rancher,rancher/rancher,rancher/rancher