[spike] QString/QByteArray: determine how missing assign() overload set should look like

See epic for more details.

The std::string::assign() overload set is a bit ... special (https://en.cppreference.com/w/cpp/string/basic_string/assign). Discuss and spec out a version of the overload set that fits more to the Qt ecosystem.

E.g.

• look at QString/QByteArray existing ctors
• consider assignment from views
  • just take views
  • or anything implicitly-convertible to them, like STL does
    • cf. QTBUG-105389 for how to implement something like this
• we probably don't want the implicit subsetting versions

Acceptance criteria:

• one assign() overload set for each of QString and QByteArray is agreed upon
• overload set contains at least std::vector's overloads (https://en.cppreference.com/w/cpp/container/vector/assign)
• document rationale for deviations from std::string's overload set (https://en.cppreference.com/w/cpp/string/basic_string/assign)