Made it possible to pass the "index_type" argument in the Index and PartialIndex constructors

[DeveloperAnonymous]

Description

Title pretty much explains it all. This is so we can set the index as unique without the need of super seeding the class

Motivation and Context

It just feels more natural to have it passed in the constructor as it is part of the index behavior to be unique or not, the type should not need to be overridden in another class

How Has This Been Tested?

This has not been tested, but I'm fairly confident in this small change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.