feat: support update for inmemory index #1725

from docarray.index import InMemoryExactNNIndex
from docarray import BaseDoc

class MyDoc(BaseDoc):
    text: str


data = [MyDoc(text=f'text {i}') for i in range(10)]
doc_index = InMemoryExactNNIndex[MyDoc]()
doc_index.index(data)

assert doc_index.num_docs() == 10

for doc in data:
    doc.text = doc.text + 'smth'

doc_index.index(data)

assert doc_index.num_docs() == 10