Implement basic SNS message filtering (#1521)

* Add support for FilterPolicy to sns subscription set_filter_attributes

* Add basic support for sns message filtering

This adds support for exact string value matching along with AND/OR
logic as described here:

https://docs.aws.amazon.com/sns/latest/dg/message-filtering.html

It does not provide support for:
- Anything-but string matching
- Prefix string matching
- Numeric Value Matching

The above filter policies (if configured) will not match messages.

moto/sqs/responses.py

@@ -30,7 +30,7 @@ class SQSResponse(BaseResponse):
     @property
     def attribute(self):
         if not hasattr(self, '_attribute'):
-            self._attribute = self._get_map_prefix('Attribute', key_end='Name', value_end='Value')
+            self._attribute = self._get_map_prefix('Attribute', key_end='.Name', value_end='.Value')
         return self._attribute
 
     def _get_queue_name(self):