AWS-StartPortForwardingSessionToRemoteHost to multiple remote hosts/ports

To facilitate testing with environments that are security group locked to the private network we port forward as per the instructions here: https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-working-with-sessions-start.html#sessions-remote-port-forwarding

```
aws ssm start-session \
--target instance-id \
--document-name AWS-StartPortForwardingSessionToRemoteHost \
--parameters '{"host":["mydb.example.us-east-2.rds.amazonaws.com"],"portNumber":["3306"], "localPortNumber":["3306"]}'
```

We have a number of services that need to be accessed and we need multiple hosts and ports forwarded via a single SSM tunnel (as we have done in the past with SSH port forwarding).

Is this supported? If not it would be a great addition.

No repository file, test, or implementation entry point is named. Start with the linked AWS Systems Manager remote port-forwarding documentation and the current AWS-StartPortForwardingSessionToRemoteHost behavior; done means establishing whether one SSM tunnel can support multiple remote hosts and ports and defining the required behavior if it can.

Written by the indexing model from the issue text.