e2e-demo: serial: Incorporate mctp-rs crate and ODP mctp relay types

[kurtjd]

The serial source currently does some manual serialization for the initial MCTP/ODP header. Worth investigating if this can be done with the existing mctp-rs crate.

[kurtjd]

Linking OpenDevicePartnership/embedded-services#715 since these are related and I will need to incorporate those changes here.