feat: add google.api_core.gapic_v1.version_header

Author: parthea

google/api_core/gapic_v1/__init__.py

@@ -18,6 +18,7 @@
 from google.api_core.gapic_v1 import method
 from google.api_core.gapic_v1 import method_async
 from google.api_core.gapic_v1 import routing_header
+from google.api_core.gapic_v1 import version_header
 
 __all__ = [
     "client_info",
@@ -26,4 +27,5 @@
     "method",
     "method_async",
     "routing_header",
+    "version_header",
 ]