feat(rds): add DatabaseProxyEndpoint L2 construct (#35064)

### Issue # (if applicable)

Closes #14186

### Reason for this change
Add DatabaseProxyEndpoint L2 construct

### Description of changes
- Add `DatabaseProxyEndpoint` L2 construct
- `DatabaseProxy` support method `addEndpoint`

### Describe any new or updated permissions being added

### Description of how you validated changes
Unit + Integ

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: phuhung273