Support building multi-platform OCI index

[rquinio]

Closes #2748

Before filing a pull request, make sure to do the following:

• Create a new issue at https://github.com/GoogleContainerTools/jib/issues/new/choose.
• Ensure that your implementation plan is approved by the team.
• Verify that integration tests and unit tests are passing after the change.
• Address all checkstyle issues. Refer to the style guide.

This helps to reduce the chance of having a pull request rejected.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication