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.

include/xrpl/basics/Expected.h

@@ -93,7 +93,7 @@ public:
     {
     }
 
-    constexpr const E&
+    constexpr E const&
     value() const&
     {
         return val_;
@@ -111,7 +111,7 @@ public:
         return std::move(val_);
     }
 
-    constexpr const E&&
+    constexpr E const&&
     value() const&&
     {
         return std::move(val_);