Adds Buildkite integration & removes CircleCI

This PR adds support for building the library in Buildkite instead of CircleCI. It also improves a few other things that we have already applied to other libraries:

5be3a8fad2649db1650df7fdfbbd9f150ecbc774 adds plugin DSL support and updates the publish-to-s3 plugin
af702cad19f61a66887d45d22ff24c99cba55008 stops tracking the .idea/compiler.xml file. We have made this change a long time ago for other projects such as this one for WordPress-Android: https://github.com/wordpress-mobile/WordPress-Android/pull/13755
During 5be3a8fad2649db1650df7fdfbbd9f150ecbc774 I've removed the deprecated kotlin-android-extensions plugin. I've considered replacing it with the newer viewBinding feature, however there were only a few usages to be updated, so I just replaced these with the regular findViewById method in cd445db670ea8b1f65d4550dc57c168a89ce63be. This is consistent with the rest of the project and admittedly since I've never worked with viewBinding feature, I didn't feel like spending extra time learning it for this.
I've removed the kotlin-parcelize plugin in 62c08a1674c93989f0b4e4a3be476766afd2aa37. I thought this was going to be necessary when I worked on 5be3a8fad2649db1650df7fdfbbd9f150ecbc774 which turned out to be incorrect.

wordpress-mobile / WordPress-Login-Flow-Android

Adds Buildkite integration & removes CircleCI #74