feat: Initial login / logout API endpoints

wp-content/themes/headless/server/api/login.gql

@@ -0,0 +1,23 @@
+fragment AuthUser on User {
+  id
+  email @nonNull
+  roles @nonNull {
+    nodes {
+      name @nonNull
+    }
+  }
+}
+
+fragment AuthPayload on LoginPayload {
+  authToken
+  refreshToken
+  user {
+    ...AuthUser
+  }
+}
+
+mutation AuthLogin($username: String!, $password: String!) {
+  login(input: { provider: PASSWORD, credentials: { username: $username, password: $password } }) {
+    ...AuthPayload
+  }
+}