[DUBBO-3137]: Seperate Constants.java into some SubConstants Class - step 1

[beiwei30]

What is the purpose of the change

issue #3137: step 1

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #4017 into master will decrease coverage by 0.01%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master    #4017      +/-   ##
============================================
- Coverage     63.47%   63.46%   -0.02%     
+ Complexity      565      564       -1     
============================================
  Files           750      751       +1     
  Lines         32415    32412       -3     
  Branches       5145     5145              
============================================
- Hits          20577    20569       -8     
- Misses         9480     9486       +6     
+ Partials       2358     2357       -1

Impacted Files	Coverage Δ	Complexity Δ
...cluster/loadbalance/ConsistentHashLoadBalance.java	87.75% <ø> (ø)	0 <0> (ø)	⬇️
...dubbo/serialize/hessian/Hessian2Serialization.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../serialize/protostuff/ProtostuffSerialization.java	60% <0%> (ø)	0 <0> (ø)	⬇️
...protobuf/support/GenericProtobufSerialization.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../apache/dubbo/remoting/transport/CodecSupport.java	77.77% <0%> (ø)	0 <0> (ø)	⬇️
...dubbo/common/serialize/kryo/KryoSerialization.java	60% <0%> (ø)	0 <0> (ø)	⬇️
...dubbo/common/serialize/java/JavaSerialization.java	80% <100%> (ø)	0 <0> (ø)	⬇️
...e/dubbo/registry/integration/RegistryProtocol.java	82.55% <100%> (ø)	0 <0> (ø)	⬇️
...mmon/serialize/hessian2/Hessian2Serialization.java	80% <100%> (ø)	0 <0> (ø)	⬇️
.../apache/dubbo/remoting/etcd/jetcd/JEtcdClient.java	9.25% <100%> (ø)	0 <0> (ø)	⬇️
... and 21 more

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[CrazyHZM]

Please take a look at my comment

[chickenlj]

LGTM