[feature] health check

[ztelur]

What this PR does:

• rename model.Cluster to model.ClusterConfig, it's static cluster configuration
• add cluster.Cluster, it's runtime cluster instance
• add health checker

[PhilYue]

@PhilYue

[codecov-commenter]

Codecov Report

Merging #421 (985541e) into develop (672beda) will increase coverage by 1.05%.
The diff coverage is 54.83%.

@@             Coverage Diff             @@
##           develop     #421      +/-   ##
===========================================
+ Coverage    36.61%   37.67%   +1.05%     
===========================================
  Files           55       55              
  Lines         3654     3740      +86     
===========================================
+ Hits          1338     1409      +71     
- Misses        2164     2174      +10     
- Partials       152      157       +5     

Impacted Files	Coverage Δ
pkg/server/cluster_manager.go	38.29% <51.72%> (+2.76%)	⬆️
pkg/config/xds/apiclient/grpc.go	17.94% <100.00%> (ø)
pkg/config/xds/cds.go	52.89% <100.00%> (+0.39%)	⬆️
pkg/server/pixiu_start.go	3.22% <0.00%> (-0.23%)	⬇️
pkg/server/listener_manager.go	0.00% <0.00%> (ø)
pkg/common/extension/filter/filter.go	15.21% <0.00%> (ø)
...oregistry/registry/zookeeper/interface_listener.go	10.00% <0.00%> (ø)
...registry/zookeeper/application_service_listener.go	0.00% <0.00%> (ø)
pkg/config/xds/lds.go	58.03% <0.00%> (+51.78%)	⬆️

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