AWS::RDS::DBInstance - Add Support for BackupTarget

### Name of the resource

AWS::RDS::DBInstance

### Resource name

_No response_

### Description

We want to provision RDS on Outposts with the option to store backup locally on Outpost. However, it seems that there is no option to enable that via CloudFormation. The feature is available for CLI, SDK and console.

We cannot update Backup Target after creation so updating the resource after provision via CFN is out of question.

Provision RDS on Outposts:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-on-outposts.creating.html

CloudFormation reference:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbinstance.html

### Other Details

_No response_

Start with the linked RDS on Outposts documentation and the CloudFormation AWS::RDS::DBInstance reference. Confirm how BackupTarget is exposed by the CLI, SDK, and console, and whether it can be represented during resource creation; done means CloudFormation can provision the requested backup target without a post-creation update.

Written by the indexing model from the issue text.