refactor: use east const convention (#5409)

This change refactors the codebase to use the "east const convention", and adds a clang-format rule to follow this convention.

src/test/app/SetTrust_test.cpp

@@ -469,7 +469,7 @@ public:
         auto& tx1 = createQuality ? txWithQuality : txWithoutQuality;
         auto& tx2 = createQuality ? txWithoutQuality : txWithQuality;
 
-        auto check_quality = [&](const bool exists) {
+        auto check_quality = [&](bool const exists) {
             Json::Value jv;
             jv["account"] = toAcct.human();
             auto const lines = env.rpc("json", "account_lines", to_string(jv));