Use bytes for raw data

[javagl]

Addressing #117 :

This replaces the use of uint8_t with std::byte for spans and vectors that contain "raw" data (file data, request/responses etc).

One could probably argue about the use of std::byte vs. unsigned chat in places where the "raw" data is assumed to be JSON data. If there is a preference for unsigned char in thsese cases, I'll review that and integrate it where it can be applied sensibly.

_{Now, let's see where CLang spills out warnings that Visual Studio did not catch...}