add support for DATAGRAM to multiplexing view

[LPardue]

it would be nice to be able to visualize how implementations make scheduling decisions between DATAGRAM and STREAM frames. This could be as simple as showing the sent DATAGRAM frame as a unique colour in amongst the STREAM frames.

[rmarx]

Talking about this more, it would be interesting to go one step further.

An HTTP/3 level extension to DATAGRAMs defines a flow-id to make it possible to group datagrams. This is very similar to a stream-id, but for datagrams. As such, it would be nice to have flows separated in the multiplexing diagram as well, as if they were streams.

This depends on how both DATAGRAM frames are reflected in qlog though, and where to look for the flow-id, as discussed in quicwg/qlog#124.