fix: add fetch strategy type

[Marsup]

This appears to be an undocumented type but it exists in https://github.com/365talents/objection.js/blob/main/lib/queryBuilder/graph/GraphOptions.js#L16-L20

[ducher]

Looks ok when checking JS code, the constant has the same values, however I got lost when trying to find the code using the options for insertGraph, too much inheritance and abstraction through classes