Include AsyncIterator import for both clients and servers (#264)

Co-authored-by: Robin Lambertz <github@roblab.la>

src/betterproto/plugin/models.py

@@ -653,7 +653,9 @@ class ServiceMethodCompiler(ProtoContentBase):
             self.output_file.typing_imports.add("AsyncIterable")
             self.output_file.typing_imports.add("Iterable")
             self.output_file.typing_imports.add("Union")
-        if self.server_streaming:
+
+        # Required by both client and server
+        if self.client_streaming or self.server_streaming:
             self.output_file.typing_imports.add("AsyncIterator")
 
         super().__post_init__()  # check for unset fields