Make the thumb shadow of CardPartMultiSlider configurable

[rcole34]

Before you make a Pull Request, read the important guidelines:

Issue Link :link:

• Is this a bug fix or a feature? **Feature!**
• Does it break any existing functionality? **No**

Goals of this PR :tada:

• Why is the change important? **Allows custom shadows on the CardPartMultiSlider thumb images**
• What does this fix?
• How far has it been tested?

How Has This Been Tested :mag:

Manually tested shadows with different configurations to confirm that it was having the desired effect

• User Interface Testing
• Application Testing

Test Configuration :space_invader:

• Xcode version: 12.4
• Device/Simulator iPhone 11 Pro
• iOS version 14.4 || MacOSX version

Things to check on :dart:

• [x] My Pull Request code follows the coding standards and styles of the project
• [x] I have worked on unit tests and reviewed my code to the best of my ability
• [x] I have used comments to make other coders understand my code better
• [x] My changes are good to go without any warnings
• [x] I have added unit tests both for the happy and sad path
• [x] All of my unit tests pass successfully before pushing the PR
• [x] I have made sure all dependent downstream changes impacted by my PR are working