Add OpenAPI Extensions support for Route documentation

[mciara]

This PR implements support for OpenAPI Extensions in route documentation, allowing developers to add custom metadata to their API routes.

Usage Example

route.extensions = mapOf(
    "x-custom-extension" to "custom-value",
    "x-another-extension" to mapOf(
        "nested" to "value",
        "number" to 42
    ),
    "x-simple-flag" to true
)

Extensibility
While this PR focuses on adding extensions support to RouteConfig, the OpenAPI specification allows extensions on many other objects. The foundation laid here can be easily extended to other configuration classes (like RequestConfig, ResponseConfig, ParameterConfig, etc.) when needed in the future.

[SMILEY4]

Hi @mciara,

Thanks you for the pr. Looks good to me 👍