feature(multipart/form-data): adding multipart/form-data helpers and file upload schema

[minolmal]

What’s Changed

This PR adds the multipartContent and multipartContentRequired used with "mulitpart/form-data" for adding content request schemas from zod-openapi. This was structured replicating existing jsonContent to keep uniformity in the library.
A FileUploadSchema was added to mostly as a demonstration.

⚠️ Lookout!

const FileUploadSchema = z.object({
  file: z.file().openapi({
    example: "test.png",
    type: "object",
  }),
});

type: "object" in .openapi({}) is crucial to properly generate openapi doc.

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This change ensures the OpenAPI examples are valid, minimal, and compatible with tools like Scalar validators. It also improves the developer experience when generating schemas with @hono/zod-openapi.

Let me know if you'd like changes, happy to adjust to fit the project direction.