feat: T-994 Update/Destroy mutations for customers

[vincent-pochet]

Add:

• updateCustomer mutation
• destroyCustomer mutation
• Render not_found error instead of nil on fetch single record queries when id does not match an existing record

[height]

This pull request has been linked to 1 task:

• T-994 [API] Add destroy and edit customer mutations in GraphQL (unlink task)

💡Tip: Add "Close T-994" to the pull request title or description, to a commit message, or in a comment to mark this task as "Done" when the pull request is merged.