[QByteArrayView] missing converters

`QByteArray` has a few converter functions like `toBase64`, `toDouble` etc. it would be great if these converters could also be supported by `QByteArrayView`.

this would allow users to use `QByteArray` converters on byte arrays, which are not owned by `QByteArray`. e.g. i'm thinking about something like:

auto base64(const char *data, size_t size)
{
    return QByteArrayView{data, size}.toBase64();
}