Refactor the MsQuic datapaths

[guhetier]

MsQuic has multiple datapath, to support different platforms (Windows UM, Windows KM, Linux...) and transport technologies (XDP, IO uring...).

This lead the datapath layers to grow in a convoluted way, hard to understand and modify. The datapath and other platform abstraction being grouped in a single lib also make it hard to unit test other MsQuic components.

We should refactor the datapath to enable:

• better compartmentalization of each datapath implementation
• better testability of the core layer in isolation
• easier development of new datapaths (fake datapaths for testing / experiement over other transport technologies...)