add: pallet-scheduler in demo runtime

[AmbientTea]

Description

This is something @Klapeyron mentioned as being useful. This pallet allows scheduling extrinsics to be run in the future

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff