[PM-18523] Add SSO external ID visibility feature flag (#5559)

Author: r-tome

src/Core/Constants.cs

@@ -108,6 +108,7 @@ public static class FeatureFlagKeys
     public const string LimitItemDeletion = "pm-15493-restrict-item-deletion-to-can-manage-permission";
     public const string PushSyncOrgKeysOnRevokeRestore = "pm-17168-push-sync-org-keys-on-revoke-restore";
     public const string PolicyRequirements = "pm-14439-policy-requirements";
+    public const string SsoExternalIdVisibility = "pm-18630-sso-external-id-visibility";
 
     /* Tools Team */
     public const string ItemShare = "item-share";