firebase-ios-sdk

 Eliminate 'instanceof' checks for FSTFieldValue  (#2651)

Previously, we checked for FSTFieldValue subtypes via:
`[value isKindOfClass:[FSTStringValue class]]`

Now, we check via a new type parameter on FSTFieldValue, i.e.:
`value.type == firebase::firestore::model::FieldValue::Type::String`

Or with the appropriate using declaration:
`value.type == FieldValue::Type::String`

This should enable easier transition to C++.