modem: hl7800: add TLS socket offloading

[bsphere]

the modem driver doesn't support offloading of TLS sockets and certificate management,
the modem itself has a cipher suite and a certificate manager controlled by AT commands.

I see that other modem drivers such as u-blox sara r4 has support for such offloading.
adding such support will both simplify network applications and reduce the memory footprint.

[rerickson1]

Feel free to submit a PR for this, but I have no plans to do this work.
The HL7800 is very limited in how many certificates it can support and how large they are. I find it too limiting for most use cases I have come across.