[mojo] Add GenericInterface_ to interface class variants

mojo/public/tools/bindings/generators/cpp_templates/interface_declaration.tmpl

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
 class {{interface.name}}Stub;
 
 class {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
 class {{interface.name}}ResponseValidator;
 {%- endif %}
 
 class {{interface.name}} {
  public:
   static const char Name_[];
   static const uint32_t Version_ = {{interface.version}};
   static const bool PassesAssociatedKinds_ = {% if interface|passes_associated_kinds %}true{% else %}false{% endif %};
 
+  using GenericInterface = {{interface|get_qualified_name_for_kind}};
+
   using Proxy_ = {{interface.name}}Proxy;
   using Stub_ = {{interface.name}}Stub;
 
   using RequestValidator_ = {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
   using ResponseValidator_ = {{interface.name}}ResponseValidator;
 {%- else %}
   using ResponseValidator_ = mojo::PassThroughFilter;
 {%- endif %}
 
@@ skipping 15 matching lines @@
 {#--- Methods #}
   virtual ~{{interface.name}}() {}
 
 {%- for method in interface.methods %}
 {%    if method.response_parameters != None %}
   using {{method.name}}Callback = {{interface_macros.declare_callback(method)}};
 {%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };