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/config.go

@@ -110,6 +110,7 @@ func New(opts Options) (*Server, error) {
 		opts:        opts,
 		DNSBLCache:  decaymap.New[string, dnsbl.DroneBLResponse](),
 		OGTags:      ogtags.NewOGTagCache(opts.Target, opts.Policy.OpenGraph),
+		store:       opts.Policy.Store,
 	}
 
 	mux := http.NewServeMux()