Add support for Querying F# discriminated union properties

[nkosi23]

This issue is to track the work on this feature discussed on discord that I am currently implementing. The goal is to make it possible to query based on the case of an F# discriminated union property. For example given the following types:

type City = { Name: string; ZipCode: string }

type Country = { Name: string; Code: string; }

type Regions =
    | Country of Country
    | City of City
    | Continent of Country list

type Survey = { Id: Guid; Topic: string; Target: Regions }

Which are persisted to the following json:

{
    "Id: "99bc3539-9928-4abe-9ea9-1bd86de7eff6",
    "Topic": "Music preferences",
    "Target" : {
        "Case": "City",
        "Fields": {
            "Item": {
              "Name": "Charlotte",
              "Code": "28500"
            }
        }
    }
}

The goal is to add a Query API of the form .WhereDiscriminatedUnionHas<TDocument, TUnion, TCase>(Func<TDocument, TUnion> lambda, Func<TCase, bool>) like the below:

Query<Survey>().WhereDiscriminatedUnionHas<Survey, Regions, Country>(x => x.Target, x => x.Code == "US")

Query<Survey>().WhereDiscriminatedUnionIs<Survey, Country, Regions>(x => x.Target)

Items to deliver

• [ ] Build the feature by implementing specialized IQueryableMember / IMemberSource
• [ ] Automatically add indexes for properties of type F# discriminated unions to select based on the case more efficiently
• [ ] Add an API to allow users to define computed indexes for specific discriminated union cases (to enable querying efficiently on case-specific properties)
• [ ] Add the query extension method: .WhereDiscriminatedUnionHas<TDocument, TUnion, TCase>(Func<TDocument, TUnion> propertySelector, Func<TCase, bool> predicate)
• [ ] Add the query extension method: .WhereDiscriminatedUnionIs<TDocument, TCase, TUnion>(Func<TDocument, TUnion> propertySelector)
• [ ] Nice to have: support arbitrary JSON serialization format instead of restricting users to persist DUs using a specific format (will need to think through how to pull that one off without introducing a dependency on a specific serialization library while avoiding reinventing the serialization world in a poor manner)

Todo

• [ ] Add Linq tests to LinqTests/Acceptance/discriminated_unions_where_extensions.cs
• [ ] Add index creation tests to MartenRegistryTests.cs