fix(rds): RDS Parameter Group doesn't support custom removal policy (#…

…28660)

>  Can't destroy a stack that includes a rds database and rds parameter group where the database has removalPolicy property set to RemovalPolicy.RETAIN

### The following is the current behaviour:
```
const parameterGroup = new ParameterGroup(this, 'ParameterGroup', {
    ...
}

const database = new DatabaseInstance(this, 'DatabaseInstance', {
    parameterGroup: parameterGroup,
    removalPolicy: RemovalPolicy.RETAIN,
    ...
})
```

When destroying the stack
```
When I destroy this stack I see the following errors:

2:04:24 PM | DELETE_FAILED        | AWS::RDS::DBParameterGroup                  | ParameterGroup5E32DECB
One or more database instances are still members of this parameter group xxx-database-parametergroup5e32decb-daetrwpaqpgw, so the group cannot be deleted (Service: Rd
s, Status Code: 400, Request ID: 389b18db-ea82-482b-a0e6-f64887da6f82)

2:19:21 PM | DELETE_FAILED        | AWS::EC2::SecurityGroup                     | DatabaseInstanceSecurityGroup8BDF0112
resource sg-0bfc8aacb3d3e3d4a has a dependent object (Service: AmazonEC2; Status Code: 400; Error Code: DependencyViolation; Request ID: 1eac5393-83df-48cf-bd75-41f25abb04
7a; Proxy: null)

```

As pointed out in the issue linked below, we cannot simply use the clusterRds' or instanceRds' removal policy because the parameter group can be simultaneously binded to a cluster and an instance. 

### New behaviour:
Add an optional property `removalPolicy` to the L2 Parameter Group resource and set the deletion policy to the generated L1 Parameter Group (Either cluster or instance) depending on the usage. 

Added unit test and integration test to verify that it works as expected.

Closes #22141

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