Adds @odata.type property and makes this a required property in schemas that have discriminator objects

[irvinesunday]

Fixes #243

This PR does the following:

Given an example of this schema, which is a base type, which exposes the discriminator mappings as shown:

The following is accomplished:

1. Adds the @odata.type property in the properties' object
2. Marks the @odata.type property as required
3. In the @odata.type property, sets the default value to '#microsoft.graph.entity
4. Updates tests to validate the above.

After the above, the schema should appear as below: