Support OpenApi Schema generation

[Swoorup]

It would be nice if we could add OpenApi 3.0 schema generation. So F# types could directly be exposed in REST models. Its possible to override schema using SchemaFilter in Swashbuckle.

[Swoorup]

I am hacking out a initial implementation like this:

        interface ISchemaFilter with
            member this.Apply (schema: OpenApiSchema, context: SchemaFilterContext) =
                let objType = context.Type
                if objType |> IsUnion then
                    schema.Properties <- Dictionary()
                    
                    let cases =
                        GetUnionCases objType
                        |> Seq.map (fun field ->
                            let case = OpenApiSchema()
                            case.Title <- field.Key
                            case
                            )
                        |> BCLConverters.seqToList
                    
                    let obj = OpenApiSchema()
                    obj.Type <- "object"
                    obj.OneOf <- cases
                    
                    let caseProp = OpenApiSchema(Type = "string")
                    
                    schema.Properties.Add("Case", caseProp)
                    schema.Properties.Add("Fields", obj)
                    ()
                

Which generates the following:

However the code is duplicated. Any ideas so it can be unified across the serializer as well as openapi generation?

[xperiandri]

@Swoorup maybe it should go to Microsoft.OpenAPI package?