Implement VPC endpoints for AWS services

[mcdonnnj]

🚀 Feature Proposal

I propose that we add VPC endpoints for AWS services to this configuration as we did in cisagov/cool-assessment-terraform#93.

Motivation

This would improve the security of this configuration by ensuring that AWS endpoint traffic can never traverse the public internet.

Pitch

This would update this configuration to align more with "lessons learned" in architecting the COOL.