Allow index and term to be optional in pico.cas

pico.cas is right now the main way to submit dml operations to picodata. Therefore we should allow for the user to use it with as less friction as possible - ignoring advanced features. I suggest to change Lua API pico.cas function to make index and term arguments optional. If not supplied they can be set to current index and term on the instance that receives this request.

We have a similar implementation in our Python API already.