QString: add missing STL-style assign(It, It) with value_type_t<It> in {char, char8_t} (i.e. UTF-8)

This is a follow-up task on QTBUG-106198.

QString::assign() can currently handle value_type's that correspond to UTF-16 and UTF-32-encoded data. It can not handle (SFINAEs out) UTF-8-encoded data, as that requires to keep state between input code points, and qstringconverter.h is currently not #include'able from qstring.h. The contiguous_iterator case is actually handled by the QAnyStringView overload, and would work if not SFINAEd out. It's SFINAEd out to keep the condition (and docs) sane. It's the non-contiguous_iterator case that's the problem.

Acceptance criteria:

• QString::assign() accepts input_iterator sequences
• correctly decodes UTF-8 to UTF-16