Add merge_headers option for Amazon SES

Add new `merge_headers` message option 
for per-recipient headers with template sends. 

* Support in base backend
* Implement in Amazon SES backend
  (Requires boto3 >= 1.34.98.)

---------

Co-authored-by: Mike Edmunds <medmunds@gmail.com>

anymail/backends/test.py

@@ -147,6 +147,9 @@ class TestPayload(BasePayload):
     def set_merge_data(self, merge_data):
         self.params["merge_data"] = merge_data
 
+    def set_merge_headers(self, merge_headers):
+        self.params["merge_headers"] = merge_headers
+
     def set_merge_metadata(self, merge_metadata):
         self.params["merge_metadata"] = merge_metadata