Parallelize certificate chain verification in client

[jpraynaud]

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