Add Jib Ownership extension for Gradle

[chanseokoh]

Gradle counterpart of the Ownership extension. (Maven extension: #1)

pluginExtension {
  implementation = 'com.google.cloud.tools.jib.gradle.extension.ownership.JibOwnershipExtension'
  configuration {
    rules {
      rule {
        glob = '/app/classes/**'
        ownership = '300'
      }
      rule {
        glob = '/static/**'
        ownership = '300:500'
      }
    }
  }
}

[loosebazooka]

ah since we provide this?

[chanseokoh]

Yeah... it's like cheating as we know Jib's implementation details. I don't expect the extension dev to do this but use implementation (and hope their specified Guava version doesn't conflict with ours). That said, I'm still unsure if we shouldn't say implementation.

[chanseokoh]

I'll merge this now to start kicking release, but feel free to bring this up later for discussion.