Added SPM support

[calebkleveter]

Issue Link https://github.com/intuit/CardParts/issues/172

Closes https://github.com/intuit/CardParts/issues/172

Goals of this PR :tada:

Adds support for the Swift Package Manager, as an alternative to CocoaPods.

How Has This Been Tested :mag:

Adds a single test CardPartImageViewTests.testAssetResources to check that resources are properly loaded in an SPM environment.

Test Configuration :space_invader:

• Xcode version: 12.0 (12A7209)
• Device/Simulator: N/A
• iOS version: N/A

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
• [ ] 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

[calebkleveter]

I am getting this warning on all the dependency modules (RxSwift, RxRelay, RxCocoaRuntime, RxCocoa, Differentiator, and RxDataSource).

The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 14.0.99.

I don't know if there is a good way to fix this, but I am looking.

[calebkleveter]

The warnings are apparently caused by Xcode 12 dropping support for iOS 8. This means that the warnings that this PR introduces are caused by the dependencies, not the package itself. Separate PRs would have to be submitted to the dependency repos to increase their minimum iOS version to 9.