Add a gateway client SDK for Kapua

[ctron]

This PR brings in a client SDK which allows to connect to Kapua in a modular way, using different transport mechanisms and communication profiles.

[codecov]

Codecov Report

Merging #701 into develop will decrease coverage by 0.11%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #701      +/-   ##
=============================================
- Coverage      54.85%   54.73%   -0.12%     
  Complexity       370      370              
=============================================
  Files            799      799              
  Lines          18735    18735              
  Branches        1704     1704              
=============================================
- Hits           10277    10255      -22     
- Misses          7787     7810      +23     
+ Partials         671      670       -1

Impacted Files	Coverage Δ	Complexity Δ
...pse/kapua/transport/mqtt/MqttClientErrorCodes.java	0% <0%> (-100%)	0% <0%> (ø)
...ipse/kapua/transport/mqtt/MqttClientException.java	0% <0%> (-28.58%)	0% <0%> (ø)
...ransport/mqtt/pooling/PooledMqttClientFactory.java	75.86% <0%> (-10.35%)	0% <0%> (ø)
...a/org/eclipse/kapua/transport/mqtt/MqttClient.java	53.57% <0%> (-5.96%)	0% <0%> (ø)
.../broker/core/plugin/KapuaSecurityBrokerFilter.java	72.19% <0%> (+0.51%)	0% <0%> (ø)	⬇️

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[hekonsek]

+1 for merging. I especially like the Kura end-to-end example :) .

[ctron]

If there are no objections I would like to merge this early next week.

[ctron]

So as just discussed on the call, we will have a more detailed discussion about that next week in London.