Move GraphQLEnumConverter from LocalClient to Client and add as default Converter

[boexler]

Actually it is not possible to pass enums as variables, because they are serialized as int and not string.

To get it back working:

1. Move GraphQLEnumConverter https://github.com/graphql-dotnet/graphql-client/blob/master/src/GraphQL.Client.LocalExecution/GraphQLEnumConverter.cs to base library
2. add the GraphQLEnumConverter to GraphQLHttpClientOptions.NewtonsoftJsonSerializer as default

new NewtonsoftJsonSerializer
{
	JsonSerializerSettings =
	{
		Converters = new List<JsonConverter>
		{
			new GraphQLEnumConverter()
		}
	}
}