Integrate Gemini Client with Cloud Firestore

[hoanghm]

• [x] Connect the Python server to Cloud Firestore
• [x] Create some sample users with information, then make sure generate_new_missions query the information for these users from Cloud Firestore.
• [x] Add a method to add new missions to the database
• [x] Make sure before new missions are returned in response that they were added to the database.
• [x] Populate some dummy data to User table and Mission table (so that there is at least one user who has had some missions in the past, and we shouldn't generate the same one for "ongoing" + avoid generate very similar one for "weekly")

[hoanghm]

• Cloud Firestore connected.
• Created get_user_by_id(user_id: str) method that will return everything about an user as a dict given their id
• Created add_mission_to_db(missions: dict, user_id:str) method that will:
  • Format each mission to be consistent with our database schema (e.g. adding status, format each step to a child object instead of just a string, etc.)
  • Add the generated mission to our database.

[hoanghm]

Whenever new missions are generated, past/existing missions already assigned to a user will also be included in the prompt to encourage the Gemini API to generate missions different than those already generated.