Change `dbsubnetgroup.DBSubnetGroup` from `Listing<String|Mapping>` to `Listing<String|Mapping>|Mapping`

Currently, it doesn't support this kind of use:
```pkl
Resources {
["SubnetGroup"] = new dbsubnetgroup.DBSubnetGroup {
// ...
SubnetIds = cfn.Ref("SubnetsIds")
// ...
}
}
```
It forces you to receive multiple subnet parameters to be passed directly in a listing, like this:
```pkl
Resources {
["SubnetGroup"] = new dbsubnetgroup.DBSubnetGroup {
// ...
SubnetIds = new Listing {
cfn.Ref("SubnetId1")
cfn.Ref("SubnetId2")
cfn.Ref("SubnetId3")
}
// ...
}
}
```
The idea is to fix it and allow using refs directly

Locate the declaration of dbsubnetgroup.DBSubnetGroup and inspect how its SubnetIds type is defined. Compare it with the two usage examples in the issue; done means a direct cfn.Ref("SubnetsIds") assignment is accepted while the existing listing form remains supported.

Written by the indexing model from the issue text.