Add support for utf8 type (strings) and byte type (byte arrays)

[samcrow]

This was inspired by https://github.com/OpenCyphal/pydsdl/pull/97 .

[pavel-kirienko]

FYI this is waiting for Nunavut to support utf8 in its C generator (at least). Once done, the PyDSDL changeset will be accepted and the Specification updated accordingly.