Fix DataServiceContext can't work with POCO's due to Missing IEdmModel

[marabooy]

Issues

*This pull request fixes issue #1003 *

Description

Introduces a new action to load the metadata from the metadata Url and set the ServiceModel. The new action runs to load the metadata if the metadata had not been set before (directly by calling context.Format.UseJson(ServiceModel) or using a user provided Hook context.Format.LoadServiceModel.
This would allow people to use the syntax below to use this method in their code when not using the code generation tools or when they want to use the latest model in their code. E.g when developing new API's or when working on API's that do not publish breaking changes.
This does not negatively impact client code generated using the tools (codegen and connected service) as they provide the ServiceModel directly or provide a Hook using the exposed context.Format.LoadServiceModel property. Existing users of OData Client also needed to have the service model set in order for it to function properly.

// POCO class
public class Person
{
        public string Id { get; set; }
        public string UserName { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
}
// Dataservice context
 public class Container : DataServiceContext
{
 public Container(Uri serviceRoot) : 
                base(serviceRoot)
        {
              this.People = base.CreateQuery<Person>("People");
        }
   public DataServiceQuery<Person> People {get; private set;}
}

...
// usage
var context = new Container(uri);
var people= context.People;

 foreach (var person in people)
 {
           Console.WriteLine(person.UserName);
  }

Checklist (Uncheck if it is not completed)

• Test cases added
• Docs added
• Build and test with one-click build and test script passed

Additional work necessary

Docs Needed

• Update the dataservice context usage docs to reflect how to use the DataServiceContext without using codegen tools.
• Update documentation to show how to override this behaviour for running test cases or implementing custom logic using the two methods highlighted in the earlier section LoadServiceModel and UseJson when one wants to ship the EDMX model together with the client, e.g in Codegen