firebase-ios-sdk

Refactor Filters (#3359)

* Rename relation_filter.{h,cc} to field_filter.{h,cc}

* Rename RelationFilter to FieldFilter

* Move Filter::Create to FieldFilter

* Port api::Query::ValidateNewFilter

* Add ArrayContainsFilter

* Add KeyFieldFilter

* Make FieldFilter constructor protected

* Port FieldFilter refactor from the JS SDK

https://github.com/firebase/firebase-js-sdk/pull/1894

* Add spec test from firebase/firebase-js-sdk#1913

* Accept array-contains as a synonym for array_contains