Add option to map field names

[guenth39]

Description

I would like to have the option to map the field names from firestore to other field names in the typsense collection. As an example, the field id is called id in firestore and typsense, but the firestore field ownerId should be called userId in typsense. To achieve this, you could modify the field Firestore Collection Fields in the typsense extension configuration to accept the following syntax and of course update the sync cloud functions to handle it.

# Previously 
id, ownerId, date

# Future 
id, ownerId=userId, date