Add native OpenAPI importer

[gschier]

Adds a native OpenAPI importer so OpenAPI files no longer have to round-trip through the Postman->OpenAPI converter path. This improves large-schema imports as well, which used to fail.

• Parse OpenAPI/Swagger documents directly into Yaak workspaces, folders, requests, parameters, headers, bodies, descriptions, and auth.
• Add focused native importer tests plus real-world fixture snapshots for APIs.guru, httpbin, NASA APOD, and XKCD.
• Add legacy converter snapshots so imported data can be compared against the old Postman-based path during review.
• Increase the plugin import wait timeout so large imports like Cloudflare can complete through the CLI/app import path.

Feedback: https://yaak.app/feedback/posts/problems-with-big-schemas