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/jtx/impl/amount.cpp

@@ -54,7 +54,7 @@ PrettyAmount::operator AnyAmount() const
 
 template <typename T>
 static std::string
-to_places(const T d, std::uint8_t places)
+to_places(T const d, std::uint8_t places)
 {
     assert(places <= std::numeric_limits<T>::digits10);
 

src/test/jtx/impl/mpt.cpp

@@ -83,7 +83,7 @@ MPTTester::MPTTester(Env& env, Account const& issuer, MPTInit const& arg)
 }
 
 void
-MPTTester::create(const MPTCreate& arg)
+MPTTester::create(MPTCreate const& arg)
 {
     if (id_)
         Throw<std::runtime_error>("MPT can't be reused");
@@ -413,7 +413,7 @@ MPTTester::getFlags(std::optional<Account> const& holder) const
 }
 
 MPT
-MPTTester::operator[](const std::string& name)
+MPTTester::operator[](std::string const& name)
 {
     return MPT(name, issuanceID());
 }