Datastore: Add unverify_user Method

[Samk13]

❤️ Thank you for your contribution!

Description

• Provides a programmatic way to revert user verification.
• Avoids the need for direct database modifications.
• Enables correction of verification errors from the admin panel.
• partially closes User Management: Missing Unverify Option After User Verification invenio-administration#202

Checklist

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[ntarocco]

I think that @alejandromumo worked on this ☺️