Finalize 6.7 API review of QCborStreamReader

As per discussion on https://codereview.qt-project.org/c/qt/qtbase/+/528397/9/src/corelib/serialization/qcborstreamreader.h#149

• the toStringish additions are different from e.g. toInteger in that they mutate the input, and are therefore correctly not const
• this raises the questions on whether they should have a different naming pattern to make that difference clearer
• since there already is readString (which perhaps would better be called readStringChunk, but that ship has sailed), we can't use that again
• readFullStringish() , readAllStringishChunks(), consumeStringish() are perhaps names that would highlight the difference to the toX() const getters as well as to the existing readStringish()

Also not concluded: naming of appendToStringish(), which also consumes the data from the stream and are therefore not const. Depending on the above, a consistent name might be readAndAppend/consumeAndAppendStringish.

Maintainer's call to conclude this. Concerns raised are valid. Nevertheless, leaving things as they are and closing this ticket as "Done" is an acceptable solution.