Improve generation parameter passing in orchestrator server

[gkumbhat]

Description

Currently we are manually re-defining the parameters required by the text generation API manually here. This can be fragile and limiting if TGIS API changes for these parameters, or if they add new parameter etc.

It would be good to use generation proto def directly to define that part of the API, or take that in as a JSON object and pass it along as is to underlying generation server. This would avoid us getting in the middle of the generation parameters we can accept.