Add DSDL string support

[pavel-kirienko]

Context from Matrix:

Pavel: Would someone volunteer to add support for strings to the Nunavut C and C++ generators? OpenCyphal/pydsdl#97
Scott: I can't this week. Perhaps next?

We should aim to implement this on a branch before merging the linked PR.

[aentinger]

@thirtytwobits maybe now's the time? (Only justification: I need this 😁 🙇 🙏 ).

[thirtytwobits]

It is a priority but it's just behind some work I'm trying to close out on libcyphal. I need to get a good block of time to do a bunch of upgrades to nunavut with the aim of getting c++ out of "experimental" and I'll include this in that work.