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/challengetest/challengetest_test.go

@@ -0,0 +1,7 @@
+package challengetest
+
+import "testing"
+
+func TestNew(t *testing.T) {
+	_ = New(t)
+}