AdvancedSearch: Hide indexed properties from the shop-api

[Draykee]

This feature request is based on this issue for elasticsearch: vendure-ecommerce/vendure#1181

Is your feature request related to a problem? Please describe.
I want to map and search for some custom products properties without exposing the exact value to the shop-api api.

Usecases:

• A hidden rank/weight that is applied to promote some products
• A hidden product owner id (I never want to expose user ids)
• Exact product locations

Describe the solution you'd like
Extend the custom mapping type with a new field:

type CustomMappingDefinition<Args extends any[], T extends CustomMappingTypes, R> = {
    public?: boolean; // **NEW** ( should default to true )
}