aws-rds: add support for adding secrets to an RDS Proxy

### Describe the feature

I would like to be able to add secrets to an RDS Proxy after the proxy has been created in an easy to use way.

### Use Case

We have an RDS stack and have multiple Application stacks that use SSM exported parameters from the RDS Stack.
The RDS Proxy is created in the RDS stack before any application stack exists, so only one secret is known to the proxy at creation time (the one for the proxy itself).

When we create an application stack we would like to get that RDS Proxy and call a addSecret method on it to add the RDS Secret that is created for the application user.

### Proposed Solution

The DatabaseProxy could add a method addSecret(ISecret) that adds the secret to the proxy and grants read access from the proxy to the secret.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.91.0

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

Linux

Start at the TypeScript DatabaseProxy construct for aws-rds and inspect how its existing secret is configured and granted access. Done means exposing an addSecret(ISecret)-style operation that attaches an application secret after proxy creation and grants the proxy read access, with coverage for this use case.

Written by the indexing model from the issue text.