feat: get issuer from context for device auth (#363)

* feat: get issuer from context for device auth

* use distinct UserFormURL and UserFormPath

- Properly deprecate UserFormURL and default to old behaviour,
to prevent breaking change.

- Refactor unit tests to test both cases.

* update example

example/server/exampleop/op.go

@@ -107,7 +107,7 @@ func newOP(storage op.Storage, issuer string, key [32]byte) (op.OpenIDProvider,
 		DeviceAuthorization: op.DeviceAuthorizationConfig{
 			Lifetime:     5 * time.Minute,
 			PollInterval: 5 * time.Second,
-			UserFormURL:  issuer + "device",
+			UserFormPath: "/device",
 			UserCode:     op.UserCodeBase20,
 		},
 	}