Implement `updateMyItem` mutation

[carlqt]

Description

We need to create a graphql mutation that updates the Item node of our graphql. This mutation will be used by the frontend to update the fields of an item in the receipt.

What makes this a little challenging is that we need to make sure that only the owner of the Receipt is able to use this mutation.

Notes

ER Diagram of the database

JWT is required to make authenticated requests. You can use it to identify the current logged-in user which opens you up for database lookups. For reference, see how the CreateReceipts mutation is implemented.