Add ExtendedCall to PLCObject as a replacement of RemoteExec and reflect it in eRPC interface.

ExtendedCall is a generic remote procedure call that runtime code can register to and that IDE extensions can call.
For example a fieldbus extension can use ExtendedCall to browse the fieldbus devices connected to the runtime.

<?xml version='1.0' encoding='utf-8'?>
<OPCUAClient xmlns:xsd="http://www.w3.org/2001/XMLSchema" Server_URI="opc.tcp://127.0.0.1:4840/freeopcua/server/">
  <AuthType>
    <x509 Certificate="my_cert.der" PrivateKey="my_private_key.pem">
      <Policy/>
      <Mode/>
    </x509>
  </AuthType>
</OPCUAClient>