Add the ability to prioritize sending messages

[bakul14]

⚡ Feature Request

It is necessary to implement setting the priority subject and service messages, since this feature is available in libcanard. Here is a piece of implementation of the publish(T const & msg) method. Priority is not a parameter and it is set to CanardPriorityNominal.

template<typename T>
bool Publisher<T>::publish(T const & msg)
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic"
  CanardTransferMetadata const transfer_metadata =
  {
   .priority       = CanardPriorityNominal,
    .transfer_kind  = CanardTransferKindMessage,
    .port_id        = _port_id,
    .remote_node_id = CANARD_NODE_ID_UNSET,
    .transfer_id    = _transfer_id++,
  };
#pragma GCC diagnostic pop

There are 2 possible solutions:

1. Add the ability to set priority through the publisher/subscriber/client/server constructor.
2. Pass priority as a parameter to the publish() method and others.