Use Keycloak-Realm-Roles for authorization

shared/src/commonMain/kotlin/net/keinekekse/app/UserRoles.kt

@@ -0,0 +1,9 @@
+package net.keinekekse.app
+
+enum class UserRoles(val roleName: String) {
+    ADMIN("admin"),
+    BOOKS_ADMIN("books-admin"),
+    BOOKS_USER("books-user"),
+    BOOKS_EDITOR("books-editor")
+
+}