Generic Chat templating code with text/json file based config; main chat updated to drive its in-prefix, in-suffix and reverse-prompt from same; chat-apply-template equivalent c-api to allow use by other codes also

Generic Chat templating code with text/json file based config; main chat updated to drive its in-prefix, in-suffix and reverse-prompt from same; chat-apply-template equivalent c-api to allow use by other codes also #11834