PM-3231 add feature flag for vault onboarding work (#3449)

src/Core/Constants.cs

@@ -62,6 +62,7 @@ public static class FeatureFlagKeys
     public const string PasswordlessLogin = "passwordless-login";
     public const string TrustedDeviceEncryption = "trusted-device-encryption";
     public const string Fido2VaultCredentials = "fido2-vault-credentials";
+    public const string VaultOnboarding = "vault-onboarding";
     public const string AutofillV2 = "autofill-v2";
     public const string BrowserFilelessImport = "browser-fileless-import";
     public const string FlexibleCollections = "flexible-collections";