Entity-graph initialisation support in MultiIdentifierLoadAccess

Description

This seems to have been alluded to in this comment on HHH-10263:

We discussed as a team and came to the agreement that long term the best option here is to add entity-graph support to MultiIdLoadAccess.

I'm finding MultiIdentifierLoadAccess very useful - it would be great if at the same time as loading a bunch of `Leaf` objects by their IDs, we could initialise their `Branch` and `Tree` associations as well. I imagine usage something like this:

Environment

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Assignee

Unassigned

Reporter

Barry Pitman

Fix versions

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backPortable

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Priority

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Configure