Add DAG endpoint and graph shell

What does this MR do?

As part 1 of #215517 (closed), this MR adds the DAG component to the page using the pipeline_details bundle and handles fetching the data from the endpoint (passed when the component mounts). It also adds specs and error handling. This is a precursor to !31767 (closed), which adds the basic graphing functionality.

It depends on this backend MR: !31583 (merged) and this refactor: !32438 (merged)

Screenshots

There are no visible changes. This is behind a feature flag.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.