add model_info.json to models/template

[infotroph]

• We now expect all models to have a model_info.json going forward, correct? If so, we should give an example in models/template
• If it's standard for model_info.json to live in the package root (rather than inst/ like register.*.xml did), will also need to list it in .Rbuildignore to prevent "non-standard files /directories found at top level" note at check time

(Moved from #2566 (comment))

[robkooper]

the code that connects and wait for model runs (as well as advertises the heartbeat) is written in python (there are no RabbitMQ bindings). We can put the model_info.json in inst as well and just need to copy it to the right location when we build the docker file.