Add an addSecurityGroup method to the DatabaseCluster construct to allow users to use existing security groups with their cluster.

Use Case

We would like to add existing security groups to the DatabaseCluster construct.

Proposed Solution

PR: #13290

Other

Related aws-rfdk PR that would like to use this method: https://github.com/aws/aws-rfdk/pull/319/files#diff-a60b144e8d7c760174d3849f941b2db9ff8244d9d0097454f3b0a515198b8112R290-R300

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

This is a 🚀 Feature Request