Tooling: Define SwiftLint version and run on CI

[iangmaia]

Changes to define a common SwiftLint version on .swiftlint.yml and use that version on both CocoaPods and CI

This PR also uses the new run-swiftlint command on https://github.com/Automattic/a8c-ci-toolkit-buildkite-plugin (implemented on https://github.com/Automattic/a8c-ci-toolkit-buildkite-plugin/pull/79) to run SwiftLint on CI.

Testing

Make sure CI is 🟢

[dangermattic]

	1 Warning
:warning:	PR is not assigned to a milestone.

Generated by :no_entry_sign: Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22678-ec9e5a9
	Version	24.3
	Bundle ID	org.wordpress.alpha
	Commit	ec9e5a96ae4e283143f9f04020200cf6195e47e4
	App Center Build	WPiOS - One-Offs #8985

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22678-ec9e5a9
	Version	24.3
	Bundle ID	com.jetpack.alpha
	Commit	ec9e5a96ae4e283143f9f04020200cf6195e47e4
	App Center Build	jetpack-installable-builds #8015

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.