Fix typehinting for Mongo Operations Service

[Maelstromeous]

src/services/mongo/mongo.operations.service.ts

Currently we're using a feckton of any or any[] typecasts, which is breaking the point of using TypeScript. I was struggling to figure out how to supply the entity type to the function and make it return correctly, due to some weirdity with Mongo and how it returns entities not being compatible.