Add a filter slot to the dialog header

[camertron]

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What are you trying to accomplish?

This PR mirrors what we did recently for Overlay by adding a filter slot. The idea is the consumer will render a text field in here (or other filtering mechanism) as we're doing for the experimental SelectPanel component being developed in dotcom. I specifically need this functionality because I'm trying to render a SelectPanel inside a Dialog and need the Dialog to support the same API as SelectPanel and Overlay (since SelectPanels are also rendered inside Overlays).

Screenshots

Integration

No changes necessary in production.

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: 1a0bbb8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Minor

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[joshblack]

LGTM ✅