Add mutations into example rust handler

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• [x] Expose mutation over GraphQL
• [x] Modify model or add functions to handle mutation
• [x] Demonstrate returning back actual error conditions from mutations

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Add graphql mutations to rust handler example.

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@CatGarab I went back and forth on the granularity of the mutations, glad to have another opinion :smile:.

I created some more abstract return types for the mutations similar to what the Apollo blog suggested. They make things a little verbose, but I can see how they could provide more flexibility in the future and it does seem like GraphQLy.