List installers

Lists the installers configured under the authenticated customer source, together with the calling customer source itself. Call this once when a customer's credentials are configured, to discover whether that customer uses installers at all. An empty `installers` array means the customer manages all of its Fixed Locations directly. A populated array means Fixed Locations can be assigned to an installer when they are created. Fixed Location callouts are attributed automatically from the resolved site's owner. A monitoring centre may hold Fixed Locations of its own alongside those belonging to its installers. Rows returned by the Fixed Location list carry a `customerSourceId`, which you can match against the values below to tell the two apart. Installers are configured by AURA on request. This endpoint does not create them.