les/checkpointoracle: move oracle into its own package (#20508)

* les: move the checkpoint oracle into its own package

It's first step of refactor LES package. LES package
basically can be divided into LES client and LES server.
However both sides will use checkpoint package for
status retrieval and verification. So this PR moves
checkpoint oracle into a separate package

* les: address comments

les/api.go

@@ -350,5 +350,5 @@ func (api *PrivateLightAPI) GetCheckpointContractAddress() (string, error) {
 	if api.backend.oracle == nil {
 		return "", errNotActivated
 	}
-	return api.backend.oracle.config.Address.Hex(), nil
+	return api.backend.oracle.Contract().ContractAddr().Hex(), nil
 }