Add support for COS public endpoint to Airflow runtime config

[ptitzler]

This PR adds support for an optional public object storage endpoint in the Apache Airflow runtime configuration, similar to what #2887 delivers for Kubeflow Pipelines.

This PR should be merged after #2887 to resolve a conflict in packages/pipeline-editor/src/RuntimesWidget.tsx. The change in this PR must take precedence over the change in the other PR or things will break.

Runtime configuration metadata editor:

Runtime configuration list entry (no public endpoint configured)

Pipeline submission confirmation message, referencing the regular endpoint

Runtime configuration list entry (bogus public endpoint configured)

Pipeline submission confirmation message, referencing the public endpoint

What changes were proposed in this pull request?

• Extend the Airflow runtime schema to include an optional input, which specifies the public endpoint of an object storage service.
• Documentation update is delivered in Add support for COS public endpoint to Kubeflow Pipelines runtime config #2887

How was this pull request tested?

• Manual testing:
  • Airflow RTC configured without public COS endpoint
    • tested editor and RTC list view
    • tested run pipeline and export pipeline
  • Airflow RTC configured with public COS endpoint
    • tested editor and RTC list view
    • tested run pipeline and export pipeline
• Updated existing test, which failed due to new property

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