refactor: Add simple clang-tidy readability checks (#6556)

This change enables the following clang-tidy checks:
-  readability-avoid-nested-conditional-operator,
-  readability-avoid-return-with-void-value,
-  readability-braces-around-statements,
-  readability-const-return-type,
-  readability-container-contains,
-  readability-container-size-empty,
-  readability-else-after-return,
-  readability-make-member-function-const,
-  readability-redundant-casting,
-  readability-redundant-inline-specifier,
-  readability-redundant-member-init,
-  readability-redundant-string-init,
-  readability-reference-to-constructed-temporary,
-  readability-static-definition

src/test/app/HashRouter_test.cpp

@@ -231,7 +231,7 @@ class HashRouter_test : public beast::unit_test::suite
         ++stopwatch;
         // Confirm that peers list is empty.
         peers = router.shouldRelay(key1);
-        BEAST_EXPECT(peers && peers->size() == 0);
+        BEAST_EXPECT(peers && peers->empty());
     }
 
     void