Legacy ConnectionSecret refactoring

[filipcirtog]

Summary

The ConnectionSecret package was originally designed to serve multiple components, including Atlas Data Federation, Atlas Database users, and Atlas Deployments. To better align with its purpose, I have renamed ConnectionSecret to SecretService, as this name more accurately reflects its role. This change also frees up the ConnectionSecret package name, which can now be reused for experimental code in the future. I have also moved some logic only used by AtlasDatabaseUsers in that package.

Proof of Work

Checklist

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[josvazg]

LGTM! great solution