Create builders for `Completion` and `Edit` structures

[valentinegb]

This pull requests adds CompletionBuilder and EditBuilder, which allows you to create completions and edits with the builder pattern. There are no builder structures for other structures like Embeddings and Model because they don't have very many fields. Here is an example of making a Completion with this PR:

let completion = Completion::builder()
    .model(ModelID::TextDavinci003)
    .prompt("Say this is a test")
    .max_tokens(7)
    .temperature(0.0)
    .create()
    .await
    .unwrap()
    .unwrap();