Add DBSecurityGroupName to AuthorizeDBSecurityGroupIngress

This was regressed here: https://github.com/aws/aws-sdk-ruby/commit/93b6ddbd671ddcc0d43926b6762e595dcfe4b625

This should have just been made optional, not removed entirely. Currently, this fails if you try to use this option.

Example:
```
AWS::rds.client.authorize_db_security_group_ingress({cidrip: "#{ip_address}/32", db_security_group_name: "default"})
```
ArgumentError: unexpected option DBSecurityGroupName

it should have been made optional, not required