SPARK checkers should ask the SP to provide Retrieval Attestation

[bajtos]

ETA: 2023-03-31

## Tasks
- [ ] When retrieving the content, ask for CAR with metadata trailer
- [ ] Verify that the checksum in the retrieval matches the CAR data
- [ ] Include the attestation in measurements submitted to spark-api

Dependencies:

• SPARK: Make SPs participate in attestation protocol roadmap#26