(aws-rds): no way to specify CloudWatch log group retention

### Describe the feature

At present when creating RDS Proxies, there is no way to specify the retention period for the resulting log group. The log group does seem to have a predictable name (in our case it seems to default to `/aws/rds/proxy/proxy` - not sure if this is a universal or how that changes if you deploy multiple proxies in an account).

It seems that it would be relatively straightforward to allow the configuration of log group retention via a `logRetention` property similar to the one which exists for Lambda functions - this could then create a `LogRetention` resource rather than forcing the user to do it manually.

### Use Case

Most people don't want infinite log retention and the associated costs. The ability to easily set a retention period would be helpful.

### Proposed Solution

allow the configuration of log group retention via a `logRetention` property similar to the one which exists for Lambda functions - this could then automatically create a `LogRetention` resource under the hood rather than forcing the user to do it manually.

### Other Information

_No response_

### Acknowledgements

- [X] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.49.0

### Environment details (OS name and version, etc.)

N/A

Start by inspecting the RDS Proxy implementation and the existing Lambda support for the `logRetention` property and `LogRetention` resource. Confirm how proxy log-group names behave when multiple proxies are deployed, then define the property behavior and verify that the generated retention resource is created correctly.

Written by the indexing model from the issue text.