feat(lib): use new challenge creation flow

Previously Anubis constructed challenge strings from request metadata.
This was a good idea in spirit, but has turned out to be a very bad idea
in practice. This new flow reuses the Store facility to dynamically
create challenge values with completely random data.

This is a fairly big rewrite of how Anubis processes challenges. Right
now it defaults to using the in-memory storage backend, but on-disk
(boltdb) and valkey-based adaptors will come soon.

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/challenge/proofofwork/proofofwork_test.go

@@ -127,15 +127,22 @@ func TestBasic(t *testing.T) {
 			i.Setup(http.NewServeMux())
 
 			inp := &challenge.IssueInput{
-				Rule:      bot,
-				Challenge: cs.challengeStr,
+				Rule: bot,
+				Challenge: &challenge.Challenge{
+					RandomData: cs.challengeStr,
+				},
 			}
 
 			if _, err := i.Issue(cs.req, lg, inp); err != nil {
 				t.Errorf("can't issue challenge: %v", err)
 			}
 
-			if err := i.Validate(cs.req, lg, &challenge.ValidateInput{Rule: bot, Challenge: cs.challengeStr}); !errors.Is(err, cs.err) {
+			if err := i.Validate(cs.req, lg, &challenge.ValidateInput{
+				Rule: bot,
+				Challenge: &challenge.Challenge{
+					RandomData: cs.challengeStr,
+				},
+			}); !errors.Is(err, cs.err) {
 				t.Errorf("got wrong error from Validate, got %v but wanted %v", err, cs.err)
 			}
 		})