feat(api): improve auth middleware

crates/api/src/auth.rs

@@ -0,0 +1,22 @@
+pub const INTERNAL_SYSTEMID_HEADER: &'static str = "x-pluralkit-systemid";
+pub const INTERNAL_APPID_HEADER: &'static str = "x-pluralkit-appid";
+
+#[derive(Clone)]
+pub struct AuthState {
+    system_id: Option<i32>,
+    app_id: Option<i32>,
+}
+
+impl AuthState {
+    pub fn new(system_id: Option<i32>, app_id: Option<i32>) -> Self {
+        Self { system_id, app_id }
+    }
+
+    pub fn system_id(&self) -> Option<i32> {
+        self.system_id
+    }
+
+    pub fn app_id(&self) -> Option<i32> {
+        self.app_id
+    }
+}