`digest_algorithm` mismatch between Puppet master and agent breaks file idempotency

When a Puppet master and agent have mismatched digest algorithms, the master may return file metadata that that doesn't match the agent digest algorithm. This means that file contents behave like they're constantly out of sync:

Notice: /Stage[main]/Main/Node[default]/File[/tmp/unbucketed-file]/content: content changed '{md5}9cf25922ced56ba90c987e4c31c51927' to '{sha256}cf77e2dfbdb2a001f9daac15548d611b14f51f2f69793be6fd203d105d1b3e1b'

When the master can respect the digest algorithm specified by the agent, it should try to use the digest algorithm specified in the metadata request. If the agent tries to use a digest algorithm that the master does not allow, the metadata request should fail.