Support multiplexing

[MathiasKoch]

It would be awesome to eventually add support for the 3gpp ts 27.010 multiplexer protocol described in https://www.3gpp.org/ftp/tsg_t/tsg_t/tsgt_04/docs/pdfs/TP-99119.pdf

Ublox has an implementation guide for it here: https://www.u-blox.com/en/docs/UBX-13001887

Furthermore a C implementation for reference can be found here: https://github.com/particle-iot/gsm0710muxer/tree/d7f7b75e8d48af760c4f585f222287eb9eff9422

This would probably be as a separate crate in this workspace, and might require extensive changes to the ATAT crate itself, which would be fine.

Any contributions are welcomed!

[MathiasKoch]

I think the way forward on implementing this, is to do it similar to the way Extended Data Mode is implemented in https://github.com/BlackbirdHQ/ublox-short-range-rs/blob/master/ublox-short-range/src/command/custom_digest.rs