(neptune): Support IAM authentication

[christophgysin]

AWS Neptune supports IAM authentication, but the current L2 constructs don't currently support enabling it.

Use Case

Authenticate DB access with IAM.

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

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This is a 🚀 Feature Request

[github-actions]

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