fix: add typing for header_params (#17914)

Towards #17637

Author: parthea

packages/gapic-generator/gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/_shared_macros.j2

@@ -65,7 +65,7 @@ except ImportError:  # pragma: NO COVER
 
 {% macro create_metadata(method) %}
     {% if method.explicit_routing %}
-        header_params = {}
+        header_params: dict[str, str] = {}
         {% if not method.client_streaming %}
         {% for routing_param in method.routing_rule.routing_parameters %}
         {% if routing_param.path_template %} {# Need to match. #}

packages/gapic-generator/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2

@@ -54,7 +54,7 @@ except ImportError:  # pragma: NO COVER
 
 {% macro create_metadata(method) %}
     {% if method.explicit_routing %}
-        header_params = {}
+        header_params: dict[str, str] = {}
         {% if not method.client_streaming %}
         {% for routing_param in method.routing_rule.routing_parameters %}
         {% if routing_param.path_template %} {# Need to match. #}