Add support for x-extension properties in Apipie.prop

[mscoutermarsh]

In our API spec, we found the need recently to set x-extension properties for some of our downstream API spec consumers (which auto generate SDKs based on our api spec).

This PR adds support for setting x-extensions.

Example:

Apipie.prop(:password, "password", { description: "The plain text password", "x-param-sensitive": true })

"password": {
  "type": "string",
  "description": "The plain text password, available only after create",
  "x-param-sensitive": true
}