Add type three scd upserts functionality

[CommanderWahid]

Add a new functionality to perform a type 3 scd upsert on a target delta table:

1. Add type_3_scd_upsert function.
2. Add unit test for type_3_scd_upsert function.
3. Update README to include the new feature.

[MrPowers]

@CommanderWahid - thanks for submitting this PR.

I don't know much about Type 3 SCD, but I am trying to learn here.

I added some comments. Thanks for your patience here. I am going to have to learn as we go and we'll have to collaborate to make sure we get the right abstraction!