Gemini API: Create "regenerate mission" functionality

We need to be able to regenerate a mission given an ID. An optional prompt can be included to guide the mission generation process.

[ ] Implement the mission regeneration functionality with a prompt
[ ] Create a chain of prompts, make sure to at least somewhat sanitize the optional guiding prompt.
[ ] Set up an API endpoint PUT /regenerate_mission/<user_id>/<mission_id> with the body below.
```
{
"guiding_prompt": "Make the mission focus on recycling water bottles"
{
```

hoanghm / Proact