Cleanup IFieldType type in data plugin

[timroes]

Part of #61760

I think we should do some TypeScript type cleanup on the IFieldType in the data plugin. It's a very central type that is/will be used in a lot of places, and I think it's currently not typed ideal.

As far as I can see the Field class is the only implementation on it, and if we look into that, it seems that the type is overly careful:

The following properties can never be undefined and we can remove the ? from them: aggregatable, searchable, scripted, visualizable. It also seems we now have a type for format.

Also I wonder if we could provide a type for how the JSON inside the saved object looks. Since we're currently only support index patterns on the client side, we have a couple of places we load the index pattern saved object on the server manually, and parse the attributes from it. It would be good if we would have a type from the data plugin showing how the format of this is, until we're having server side index pattern support.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

Closing in favor of #107235