Add date field in templates and template parts

[ntsekouras]

What?

Extracted from: #77086

This PR adds the date field in templates and template parts. This will help with the simplification of revisions handling for these post types, as there is a lot of scattered code around that working around the fact that they expose only the modified date.

Testing Instructions

1. date field should be returned through REST API
2. everything should work exactly as before

[github-actions]

Flaky tests detected in a4b09b7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/24128552711
📝 Reported issues:

• [Flaky Test] post title and content are editable and blocks can be inserted #76524 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js
• [Flaky Test] should move focus back to the Publish panel toggle button when canceling #76179 in /test/e2e/specs/editor/various/publish-panel.spec.js
• [Flaky Test] should show correct homepage actions based on current homepage or posts page #68892 in /test/e2e/specs/site-editor/homepage-settings.spec.js

[Mamaduka]

Thanks, @ntsekouras! This looks good ✅

[github-actions]

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Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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