feat: handle removed attributes

[smomin]

• add logic to handle removed attributes from firestore record.
• add unit test for the new logic

[saifurrehmanx]

When this update will available in firebase extensions?

[Haroenv]

this makes sense to me, but why aren't we always using save object?

[smomin]

@Haroenv I am solving for race condition which is described here, https://github.com/algolia/firestore-algolia-search/issues/32. Now, using NodeJS, the race condition can be somewhat frequent to rare. The reason I did not use saveObject is because it does not provide versioning to protect against race conditions. The partialUpdate method has version which is helping to prevent the race condition. Or, I can implement an extra Firebase record update before sending it to Algolia but this will incurr a read hit.