Use sio via git submodules

[tmadlener]

Keeping the sio sources here and in the upstream repo in sync is potentially more work than it needs to be and it might be easier to fix issues like #178 in upstream and then directly pick them up here if SIO would be included as a git submodule here.

[arummler]

I would propose to avoid submodules; better to use FetchContent. Mixing git and cmake methods otherwise quickly becomes tricky. Particularly for downstream consumers.