Add encryption/decryption helpers for E2E tests involving authentication

[kdaviduik]

feat: Add ejson decryption helper for authenticated E2E tests

• Implement secure in-place decryption/encryption of secrets.ejson
• Add helper functions to extract test credentials safely
• Include withTestCredentials utility for automatic lifecycle management
• Add tests to verify ejson helper functionality
• Never create temporary files with decrypted content for security

WHY are these changes introduced?

Fixes #0000

WHAT is this pull request doing?

HOW to test your changes?

Post-merge steps

Checklist

• [ ] I've read the Contributing Guidelines
• [ ] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [ ] I've added a changeset if this PR contains user-facing or noteworthy changes
• [ ] I've added tests to cover my changes
• [ ] I've added or updated the documentation