Refactor entities to avoid repetitive entry.

Right now we have a lot of entities, each of them is a flatten dataclass. This means if we want to add/remove/change a field, we need to change many entities. This is repetitive and error-prone.

In this MR:

• The entities are refactored into a nested structure
• Some information associated with an entities are moved into the entity as a property.
• Clean up the BQ table schema: the new table schema can be found here
• Make sure that the prompt engine does not depend on prompt library.