Support multiple tags for ECR Push task

Customer request: support adding multiple tags at a time with the ECR Push task.
* Commas are allowed in tags, so a comma-separated list is not ideal
* Potentially use a textarea input and create a tag per line?

The issue names the ECR Push task but no files, tests, or entry points. Start by locating that task's tag input and existing handling, then determine how its configuration is represented and validated. Done means users can supply multiple tags, including tags containing commas, and each tag is passed to the push operation.

Written by the indexing model from the issue text.