Implement List a single todo by ID

[CBeane313]

Implement the ability to search for a specific item given it's ID, if no ID give 404 error

From LabTasks:
Implement an api/todos/58895985c1849992336c219b server-side endpoint, which returns the single todo with the given _id. It should return a 404 (use the Javalin NotFoundResponse class) if there is no todo with the specified _id.