testserver: add digest support to ArtifactServer

[hiddeco]

With the source-controller moving away from a fixed SHA-256 checksum in favor of a "digest", the ArtifactServer should support working with this as well.

Based on a quick scan through the code, the digest algorithm could either be an option on the server, or a parameter provided to the method which produces an Artifact. In any case, it should be a configurable option for the consumer of the library to allow verification against a set of algorithms the source-controller can produce (e.g. BLAKE3).