feat(openapi): merge request body, support form urlencoded parameters

[ColinFrick]

This commit introduces two new features to improve the OpenAPI generation process.

1. It adds logic to merge request bodies when combining multiple API definitions. This ensures that request bodies from both old and new API objects are properly merged, handling scenarios where they use different media types or schemas. For conflicting schemas, a oneOf node is utilized to allow multiple schema variations in a compatible manner.
2. It enhances support for form-encoded parameters by including them in the OpenAPI request body. If form parameters are present, they are added under the application/x-www-form-urlencoded content type, alongside any existing JSON body parameters, with their schema automatically generated.