Upsert objects in bulk. The upsert of an object may create a new object
or update an existing object, depending on wether it references an existing
object or not. All objects must either have an id or extId, and
they must refer to unique objects.

The importMode impacts how an existing object is updated. PUT
replaces the entire object, while PATCH only updates the parts of the
object that is in the upsert, leaving the rest of the object intact. The
default is PATCH. Note that an array will replace all values for that
array, also in PATCH mode.