RK-A2

Deploy set_prompt_mods api
1.Inputs (types): model (string), index (int), prefix (string), suffix (string)
2.store in new Datastore kind "prompt_prefixes".
3.Encrypt prefix and suffix and store in Datastore

Changes to https://call-openai-api-yhb42btdva-uc.a.run.app:
1.add parameters model and index. if these are not present, assume "gpt 3.5" and 1 as arguments.
2.retrieve prefix and suffix
2.decrypt them
3.Before sending prompt to OpenAI API, set prompt to concatenate(prefix, \n, prompt, \n, suffix)