Add Jib Ownership Maven Extension

[chanseokoh]

Towards GoogleContainerTools/jib#1257 and GoogleContainerTools/jib#1955, as well as a workaround for GoogleContainerTools/jib#1270.

The Jib Maven extension allows the user to customize file ownership in layers.

The following example sets UID 300 and GID 500 for /app/classes/**.

<configuration>
  <pluginExtensions>
    <pluginExtension>
      <implementation>com.google.cloud.tools.jib.plugins.extension.maven.ownership.JibOwnershipExtension</implementation>
      <configuration implementation="com.google.cloud.tools.jib.plugins.extension.maven.ownership.Configuration">
        <rules>
          <rule>
            <glob>/app/classes/**</glob>
            <ownership>300:500</ownership>
          </rule>
        </rules>
      </configuration>
    </pluginExtension>

However, due to the way Jib adding parent directory tarball entries, the directories /app and /app/classes will continue to have the ownership 0:0 (root:root). Considering the use-case of Skaffold, we should verify if this can cover the use-case; otherwise, we may need to revisit GoogleContainerTools/jib#1270.

[loosebazooka]

can we change the PR description to describe the ownership extension?

[loosebazooka]

I think modify technically isn't the process happening here? We're creating a new layer with changed values?

[chanseokoh]

Changed the names to applyRulesToLayer/FileEntry. That said, I changed Configuration.Entry to Configuration.Rule. The latter sounds better.

[chanseokoh]

Updated the description. Worth checking.

I'll take care of the code comments next week.