Add and/or remove links on ONE multi-link `field` of an element, leaving the
rest of that field's array untouched.
Use this for additive/subtractive link edits (unlike `update_element`, which
REPLACES a link array). `type` is a slug from `list_element_types`; `id` is the
element's UUID; `field` must be a multi-link field on that type (see
`get_element_schema(type)`). `add` is a list of UUIDs to link (each must already
exist in the world; adding an already-linked id is a no-op) and `remove` is a
list of UUIDs to unlink (removing an absent id is tolerated). Requires a WRITE
API-Key. Returns `{type, id, field, values}` where `values` is the field's full
UUID array after the edit. Errors (naming `field`) if it is not a multi-link
field of this type, if the element does not exist, or if an added target id is
absent.
Connector