Add `run_swiftlint` command

[iangmaia]

Adding a new command to run SwiftLint on an iOS repo. This is an iteration on the work first implemented on https://github.com/Automattic/pocket-casts-ios/pull/1326, moving the common SwiftLint script to this shared plugin.

The command will load the right SwiftLint version using the swiftlint_version field defined in the .swiftlint.yml file (props to @jkmassel for the idea), running SwiftLint via Docker and reporting warnings / errors using Buildkite annotations.

A setup testing the new command can be seen here.

---

• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.