[DUBBO-3137]: step3 - seperate constants for remoting

[CrazyHZM]

What is the purpose of the change

issue #3137: step 3 -remoting

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #4023 into master will decrease coverage by 0.02%.
The diff coverage is 73.5%.

@@             Coverage Diff             @@
##             master   #4023      +/-   ##
===========================================
- Coverage     63.42%   63.4%   -0.03%     
- Complexity      564     565       +1     
===========================================
  Files           755     755              
  Lines         32425   32425              
  Branches       5145    5145              
===========================================
- Hits          20567   20560       -7     
- Misses         9496    9504       +8     
+ Partials       2362    2361       -1

Impacted Files	Coverage Δ	Complexity Δ
...src/main/java/org/apache/dubbo/remoting/Codec.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...pache/dubbo/monitor/dubbo/DubboMonitorFactory.java	100% <ø> (ø)	6 <0> (ø)	⬇️
...rc/main/java/org/apache/dubbo/remoting/Codec2.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...he/dubbo/remoting/transport/netty/NettyClient.java	66.1% <ø> (ø)	11 <0> (ø)	⬇️
...ache/dubbo/common/constants/RegistryConstants.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...enter/support/nacos/NacosDynamicConfiguration.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...java/org/apache/dubbo/remoting/etcd/Constants.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...g/apache/dubbo/config/AbstractReferenceConfig.java	94% <ø> (ø)	0 <0> (ø)	⬇️
...e/dubbo/registry/integration/RegistryProtocol.java	82.55% <ø> (ø)	0 <0> (ø)	⬇️
...che/dubbo/registry/nacos/NacosRegistryFactory.java	0% <ø> (ø)	0 <0> (ø)	⬇️
... and 77 more

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[beiwei30]

LGTM

[beiwei30]

pls. remember to remove constants from Constants.java.