Add AllowClients Field to packets

[mochi-co]

Adds a new AllowClients field to the packet structure. This field can be set during the OnMessage event hook, allowing an embedding server to selectively deliver messages to one or more clients within one or more topics based on their client id. For example:

// Add OnMessage Event Hook
server.Events.OnMessage = func(cl events.Client, pk events.Packet) (pkx events.Packet, err error) {
  pkx = pk
    
  // Example of using AllowClients to selectively deliver/drop messages.
  // Only a client with the id of `allowed-client` will received messages on the topic.
  if pkx.TopicName == "a/b/restricted" {
    pkx.AllowClients = []string{"allowed-client"} // slice of known client ids
  }

  return pkx, nil
}

If AllowClients is nil or not set, it is ignored and the message is delivered as normal.

All unit tests passing, paho compatibility tests passing

[codecov-commenter]

Codecov Report

Merging #14 (fc61cc9) into master (fb13648) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   98.96%   98.96%           
=======================================
  Files          18       19    +1     
  Lines        2020     2027    +7     
=======================================
+ Hits         1999     2006    +7     
  Misses         19       19           
  Partials        2        2           

Impacted Files	Coverage Δ
server/internal/packets/packets.go	100.00% <ø> (ø)
server/internal/clients/clients.go	98.92% <100.00%> (ø)
server/internal/utils/utils.go	100.00% <100.00%> (ø)
server/server.go	100.00% <100.00%> (ø)

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