feat(aggregate): add Aggregate.prototype.finally() to be consistent with Promise API for TypeScript

[vkarpov15]

Summary

Ran into this issue earlier: using Model.aggregate() in a place where a promise is expected, like assert.rejects(), TypeScript reports an error because Aggregate doesn't have finally() or [toStringTag].

Argument of type 'Aggregate<any[]>' is not assignable to parameter of type 'Promise<unknown> | (() => Promise<unknown>)'.
      Type 'Aggregate<any[]>' is missing the following properties from type 'Promise<unknown>': finally, [Symbol.toStringTag]

We already have [toStringTag] and finally() on Query, so would be nice to have them on Aggregate as well.

Examples

[IslandRhythms]

LGTM