Add SwiftPM and Github actions support

[master-nevi]

Description

• Add a SwiftPM manifest file
• Update the .gitignore file to ignore local Swiftpm artifacts
• Add Github actions for continuous integration. Actions will take into affect immediately as shown on my fork: https://github.com/master-nevi/Override/actions
• Update README badge
• Removed the _Pods.xcodeproj symlink as it doesn't appear to be used. I can add it back but I believe it's an artifact of the original pod lib create for Carthage support which isn't referenced in the README.

NOTE: There appears to be some build issue with Nimble in the Example-Swift project and so the CI builds but is currently unable to run tests. Support from original maintainers of the library would be needed for this.

How Has This Been Tested?

With test tools described in the Github actions yaml file.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to 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 read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

License

I confirm that this contribution is made under an MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[adamkaplan]

LGTM