Support variable generation and template expansion in serverURL

[czechboy0]

We should support variable expansion in the Server Object: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#server-variable-object

The generated Servers enum already uses static methods to prepare for this:

enum Servers {
  static func server1() throws -> URL
}

Supporting variables would mean generating e.g.

enum Servers {
  static func server2(environment: String, foo: Int, bar: Components.Schemas.Something) throws -> URL
}