feat: Attempt to generate petstore server

This commit contains my attempts to generate a full server backend from the petstore OpenAPI specification.

I have been struggling to correctly handle references within the OpenAPI schema. I have tried several approaches, including:
- Matching on `SchemaKind::Any` and checking for a `reference` field.
- Using `SchemaKind::Reference`.
- Using a helper function that takes a `ReferenceOr<Schema>`.

I am currently stuck on the implementation of the `get_type_from_schema` and `get_diesel_type_from_schema` helper functions. I am not sure how to correctly handle the different variants of the `ReferenceOr` enum and the `SchemaKind` enum.

I am seeking advice on the correct way to handle references in the `openapiv3` crate.

Author: google-labs-jules[bot]

output/models.rs

@@ -1,5 +0,0 @@
-#[derive(Clone, Debug, PartialEq, serde::Deserialize, serde::Serialize)]
-pub struct DummySchema {
-    pub id: i64,
-    pub name: String,
-}