Enable adding custom extensions in the swagger's info section

[themakarrony]

Problem

Currently, Swaggo only supports adding custom extensions (x-*) at the root of the generated Swagger JSON.
However, the Swagger 2.0 / OpenAPI specification explicitly allows vendor extensions under the info object.

Expected behavior

It should be possible to add custom extensions in the info section (e.g., x-example, x-company-metadata, etc.).

Example

Given this Swagger definition:

"info": {
    "title": "My API",
    "version": "1.0",
    "description": "Demo API",
    "x-example": "custom metadata"
}

Actual behavior

Swaggo does not allow passing custom extensions to the info section.

Use case

We need to comply with internal standards (e.g., Spectral rules, company policies) that require specific metadata in the info section.