SMS-related commands

[mallenbach]

Even if this is definitely not the current technical trend, I think it might be useful to expose SMS-related commands at the API.

I can see an interest as fallback mode in case the network coverage is not good enough for a proper data connectivity. This is something I added to my developments as an extension of this library and this is used on a regular basis by the
customers.

Let me know if you want me to create a PR to share what I already developed.

[chinglee-iot]

@mallenbach
Thank you for raising this feature request for SMS functionality.
When adding new APIs to the library, we will need to design them to be modem-agnostic, supporting both 3GPP standard AT commands and modem-specific proprietary commands.
It would be great if you can create a PR to share your development.
We look forward to reviewing your PR and discussing the implementation details to ensure broad compatibility across cellular modules.