Order entity (admin) resolver misses customer @ResolveField

[mschipperheyn]

Describe the bug
The order entity resolver doesn't resolve the customer field, instead relying on the service method to retrieve the relation in certain scenario's. That leads to unexpected customer: null if you access your order in another way, e.g. as a key in a custom entity.

To Reproduce
You can see this in the entity resolver.

Expected behavior
Any relevant relation IMHO should be resolveable through on the entity resolver and not rely on a service method implementation.

This also hooks in to the desire to have a dataloader to optimize these scenario's, e.g. when you're loading the same relation in a query over and over again, in order to aboid N+1 performance loss

Environment (please complete the following information):

• @vendure/core version: 2.0.x
• Nodejs version: 20
• Database (mysql/postgres etc): postgres

Additional context