Revert "Remove unused constant"

This reverts commit 4fcb9da.

This is required to make feature flags work on the client

src/Core/Constants.cs

@@ -36,6 +36,7 @@ public static class FeatureFlagKeys
     public const string DisplayEuEnvironment = "display-eu-environment";
     public const string DisplayLowKdfIterationWarning = "display-kdf-iteration-warning";
     public const string TrustedDeviceEncryption = "trusted-device-encryption";
+    public const string SecretsManagerBilling = "sm-ga-billing";
 
     public static List<string> GetAllKeys()
     {