pycrate_osmo.SEDebugMux: add inner message definitions

[axilirator]

This PR adds known inner message definitions, which can be used to:

• identify the target device (model info, IMEI);
• obtain the list of available Data Providers;
• establish and terminate a connection to/with a Data Provider;
• ping the target.

A command line DebugMux client implementation can be found here:

https://github.com/axilirator/sedbgmux

[p1-bmu]

Thanks again.