feature: External Link (in Action Block)

[jbmoelker]

Changes

• Adds ExternalLink item to ActionBlock with required title and url field (with validation).
• Matches the openInNewTab property also available in the built-in links in StructuredText fields.
• Reuses the same style field configuration used for the InternalLink block.

Associated issue

N/A

How to test

1. Open preview link
2. Navigate to the Action Block demo and scroll down to the "Action block with External Links" section.
3. Verify the External Links render as primary and secondary
4. Verify the first action opens the external link in the same tab, and the second action opens a new tab
5. Go the the external-links environment and play around with the new External Link option in Action Blocks
6. Verify changes made are reflected in the web page (note: run locally as changes are not updated on deploy preview)

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer