feat: define administrator auth contracts

fixtures/v1/auth-login.response.json

@@ -0,0 +1,4 @@
+{
+  "session_token": "fixture-session-token-never-use",
+  "must_replace_credentials": true
+}

fixtures/v1/auth-session.authenticated.json

@@ -0,0 +1,3 @@
+{
+  "authenticated": true
+}