Adding update_index() method for similarity indexes

[brimoor]

Marking as a draft to ponder a bit more whether it makes sense to provide builtin methods to update other brain runs like compute_uniqueness(), compute_visualization(), etc.

import fiftyone as fo
import fiftyone.zoo as foz
import fiftyone.brain as fob

dataset = foz.load_zoo_dataset("quickstart")

dataset2 = fo.Dataset()
dataset2.add_collection(dataset[:100].select_fields())

index = fob.compute_similarity(dataset2)

dataset2.add_collection(dataset[100:].select_fields())

index.update_index()

# No-op
index.update_index()