Parse inverted indexes field names into array of strings

[CriggerMarg]

Right now IndexApiClient.GetAllCollectionIndexesAsync fails with deserealization issue.

It happens because for persistent indexes only field names are stored and returned as array of strings.
For inverted indexes it's different: there is an object with properties.

My goal was to fix deserealization issue to allow my work to continue as we want to use inverted indexes in our database and using mentioned method for database schema verify.

I do know there is InvertedIndexField model but don't think adding a major change to interface is what is needed for this issue.

[JLedel]

The Go sdk have two properties for each index, which I kind of like:

// RegularIndex is the regular index object. It is empty for the InvertedIndex type.
RegularIndex *IndexOptions `json:"indexes"`

// InvertedIndex is the inverted index object. It is not empty only for InvertedIndex type.
InvertedIndex *InvertedIndexOptions `json:"invertedIndexes"`

The generic properties for the index is still on the "base" model.

The Java SDK have two different methods instead, either you get normal indexes or you get inverted ones. Maybe we should discuss the best way forward in #493?

[DiscoPYF]

The Java SDK have two different methods instead, either you get normal indexes or you get inverted ones.

I like the idea of having different methods, it's very clear from a consumer pov.

We should avoid putting serializer-specific attributes in the models, because we support custom serialization implementations.

I'll reply on #493