Support including system messages in templates

[caufieldjh]

#321 concerns providing system messages, and that is added in #435.
In some cases it would be convenient to provide system messages directly within an extraction template.
Caveat: this is getting very close to just being a form of RAG, as one use of system messages is to provide context for subsequent tasks. A more detailed source of context would be related documents accessed through RAG.
Let's consider this another inflection point between OntoGPT and CurateGPT.