These allow to set a different system prompt for conversations and
one-shot prompts.
Also add a new `modelDefaults.systemPrompt` configuration key to define
the default system prompt, which can be overriden per-execution with the
--system-prompt or --system-prompt-file flags.
Add new SetStructDefaults function to handle the "defaults" struct tag.
Only works on struct fields which are pointers (in order to be able to
distinguish between not set (nil) and zero values). So, the Config
struct has been updated to use pointer fields and we now need to
dereference those pointers to use them.
