Adds discriminator object to complex types which have derived types

[irvinesunday]

Fixes #233

This PR:

• Fixes the function that fetches the derived types of structured types and adds them to the discriminator object. The fix includes retrieving the derived types of complex properties and removing the condition that checks for base types before adding the discriminator object. Discriminator objects are not only required to be added to structured types which have base types.
• Adds test to validate the above fix.
• Adds release note info.