qOffsetStringArray(): allow iteration

QOffsetStringArray is a kind of container, and it's sometimes useful to iterate over it (esp. with std::lower_bound). The value_type if the iterator should be decltype(viewAt(0)).

Acceptance criteria:

1. The following tests pass:

   constexpr auto table = qOffsetStringArray("aha", "boo", "coocoo");
   static_assert(q20::is_sorted(table.begin(), table.end()));
   auto it = std::lower_bound(table.begin(), table.end(), table.viewAt(1));
   assert(it == table.begin() + 1);
   auto rit = std::lower_bound(table.rbegin(), table.rend(), QByteArrayView{"axe"}, std::greater<>{});
   assert(rit == table.rbegin() + 2);
   

2. there is a QSortedOffsetStringArray that statically asserts that the elements are ordered and provides a member contains() with O(log N) complexity (ie. binary search, using std::lower_bound().
3. the BannedHeaders array in qnetworkreplywasmimpl.cpp is ported to QSortedOffsetStringArray and its contains() method.