feat(connectors): add base connector and registry for detection

Idea is to have a "plugin-type" system, where new connectors can extend the `BaseConnector` class and implement the fetch posts method.

These are automatically detected by the registry, and automatically used in new Flask endpoints that give a list of possible sources.

Allows for an open-ended system where new data scrapers / API consumers can be added dynamically.

server/core/datasets.py

@@ -1,7 +1,7 @@
 import pandas as pd
 from server.db.database import PostgresConnector
 from psycopg2.extras import Json
-from server.exceptions import NotAuthorisedException, NonExistentDatasetException
+from server.exceptions import NonExistentDatasetException
 
 class DatasetManager:
     def __init__(self, db: PostgresConnector):