Parallelize certificate chain verification in client

## Issue
In order to accelerate the restoration of the artifacts in the client, the certificate chain verification could be parallelized: the certificates
are individually verified and the validity of the chaining is verified separately.

**Note**:
How to make the certificate chain verifier more pure, and decouple it from the retriever:
- This would make the mithril client API more easy to use
- This would also open the door to more efficient verification (map/reduce, parallelize, ...)
- See comments in the issue https://github.com/input-output-hk/mithril/issues/1141#issuecomment-1754810473