* 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.