QCanDbcFileParser: provide API to supply file contents as a QString

After https://codereview.qt-project.org/c/qt/qtserialbus/+/486148 is merged, we expect the files to be encoded in UTF-8.

However, the users report that it's not always the case. In practice the files can be provided in any encoding (e.g. Chinese GB 18030).

The suggestion is to introduce an API that will be taking the file contents as a QString, so that the user could read the file and do all the encoding conversions in advance.

Update: the internal parser operates on QStringView, so it makes sense to have a method signature like:

bool QCanDbcFileParser::parseFromData(QStringView data);

Probably even better approach would be to convert the internal parser to QAnyStringView, and then use it in the signature... But that requires at least QRegularExpression and friends to support QAnyStringView APIs, which is not the case as of now.