Implement count in expansion options

[lenardchristopher]

Addresses #137

Note: First time touching the code of this repo. I don't see any contribution docs, so I'm not sure what is necessary.

The test I wrote fails with the following exception

Expression of type 'System.Int32' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable`1[AutoMapper.OData.EFCore.Tests.Model.ProductModel]' of method 'System.Collections.Generic.List`1[AutoMapper.OData.EFCore.Tests.Model.ProductModel] ToList[ProductModel](System.Collections.Generic.IEnumerable`1[AutoMapper.OData.EFCore.Tests.Model.ProductModel])' (Parameter 'arg0')
   at System.Dynamic.Utils.ExpressionUtils.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arguments, ParameterInfo pi, String methodParamName, String argumentParamName, Int32 index)
   at System.Linq.Expressions.Expression.Call(MethodInfo method, Expression arg0)
   at AutoMapper.AspNet.OData.Visitors.MethodAppender.VisitMethodCall(MethodCallExpression node) in /Users/chrislenard/repos/AutoMapper.Extensions.OData/AutoMapper.AspNetCore.OData.EFCore/Visitors/MethodAppender.cs:line 43
   at AutoMapper.AspNet.OData.Visitors.MethodAppender.AppendQueryMethod(Expression expression, ODataExpansionOptions expansion, ODataQueryContext context) in /Users/chrislenard/repos/AutoMapper.Extensions.OData/AutoMapper.AspNetCore.OData.EFCore/Visitors/MethodAppender.cs:line 23

I interpret this to mean that COUNT was used properly, but handling the value in the result is throwing an error. Would love some advice. Thanks.

[lenardchristopher]

@dotnet-policy-service agree

[BlaiseD]

It's probably better to place the test here. The test should confirm that the count filed exists in the result. It should fail if run against the current code.