✨ Add `Entity` model to enable `ULabel`-like experience for biological entities

[sunnyosun]

The main difference from ULabel is that Entity is coupled to Source. ULabel can not be coupled to Source as it will require moving the Source registry into lamindb.

For example, if a user wants to create a new registry called Vaccine and couple to https://obofoundry.org/ontology/vo.html.

Vaccine = bt.Entity(name="Vaccine", is_type=True).save()
source = bt.Source(name="vo", version="...", ...).save()
Vaccine.add_source(source)