Add subtitle prop to DialogHeader

[liamsms]

Closes #3800

• Added an optional subtitle prop to DialogHeader, the subtitle renders directly below the header text.
• Updated the dialog docs, storybook, and tests.

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[joshwooding]

Thanks for the contribution @liamsms. We're still looking into the requirements for this feature, but we'll keep this PR open

[joshwooding]

Thanks for the contribution @liamsms. We're still looking into the requirements for this feature, but we'll keep this PR open.

Hi @liamsms, Since then, we've decided to build a new component to accommodate feedback and other design changes for Dialog and similar structured components. Therefore, we won't be adding subtitle to the DialogHeader component. Currently, the subtitle is technically possible with composition using DialogHeader.

Thank you for your work.