change: Stake data source improvements

[AmbientTea]

Description

• changes data source method arguments from vectors to slices
• adds a mock data source
• wires the data source in the node (unused for now, will be used when the payouts pallet is implemented)
• fixes a couple of random compilation warnings caused by not using feature flags etc.

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• New tests are added if needed and existing tests are updated.
• Relevant logging and metrics added
• CI passes. See note on CI.
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG Partner Chains developers to do this
for you.