fix: validate messages against all authenticators

In commit fb08c99 where we've
migrated the haproxy-spoe-go library, a regression has been
introduced: indeed, for given received messages, all the
associated authenticator should have been validated against
before continuing.
Fix consists on iterating on all the messages and associated
authenticator.

internal/agent/agent.go

@@ -41,7 +41,6 @@ func StartAgent(interfaceAddr string, authenticators map[string]auth.Authenticat
 				if isAuthenticated {
 					authenticated = true
 				}
-				break
 			}
 		}