Align 2.7.5: Event dispatch service discovery

[Patrick0308]

What this PR does:
Align 2.7.5: Implement of Event dispatch service discovery. It will be used in Service Discovery Registry.
Which issue(s) this PR fixes:
This close #450

[codecov-io]

Codecov Report

Merging #487 into feature/dubbo-2.7.5 will decrease coverage by 0.14%.
The diff coverage is 50.00%.

@@                   Coverage Diff                   @@
##           feature/dubbo-2.7.5     #487      +/-   ##
=======================================================
- Coverage                66.81%   66.66%   -0.15%     
=======================================================
  Files                      196      199       +3     
  Lines                    10125    10212      +87     
=======================================================
+ Hits                      6765     6808      +43     
- Misses                    2708     2751      +43     
- Partials                   652      653       +1     

Impacted Files	Coverage Δ
common/observer/event.go	16.66% <0.00%> (ø)
...istry/common/event_publishing_service_discovery.go	40.00% <40.00%> (ø)
registry/common/service_discovery_event.go	66.66% <66.66%> (ø)
registry/common/service_instance_event.go	71.42% <71.42%> (ø)
protocol/dubbo/client.go	67.87% <0.00%> (-1.22%)	⬇️
remoting/kubernetes/watch.go	78.30% <0.00%> (-0.95%)	⬇️
remoting/kubernetes/listener.go	56.07% <0.00%> (ø)
... and 1 more

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[fangyincheng]

LGTM