[Enhancement] JsonRpcProtocol support Generalization #4595

[CrazyHZM]

What is the purpose of the change

JsonRpcProtocol support Generalization #4595

Brief changelog

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Verifying this change

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[codecov-io]

Codecov Report

Merging #4596 into master will decrease coverage by 0.01%.
The diff coverage is 46.8%.

@@             Coverage Diff              @@
##             master    #4596      +/-   ##
============================================
- Coverage     63.96%   63.94%   -0.02%     
- Complexity      452      457       +5     
============================================
  Files           769      771       +2     
  Lines         33206    33248      +42     
  Branches       5237     5241       +4     
============================================
+ Hits          21239    21261      +22     
- Misses         9546     9561      +15     
- Partials       2421     2426       +5

Impacted Files	Coverage Δ	Complexity Δ
.../dubbo/rpc/protocol/http/JsonRemoteInvocation.java	0% <0%> (ø)	0 <0> (?)
...bbo/rpc/protocol/http/JsonRpcProxyFactoryBean.java	46.66% <46.66%> (ø)	3 <3> (?)
...g/apache/dubbo/rpc/protocol/http/HttpProtocol.java	51.25% <68.18%> (+5.21%)	8 <4> (+2)	⬆️
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0%> (-5.27%)	0% <0%> (ø)
...e/dubbo/remoting/transport/netty/NettyChannel.java	55.68% <0%> (-4.55%)	20% <0%> (-1%)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	80% <0%> (-3.34%)	0% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	64.77% <0%> (-2.28%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	67.87% <0%> (-1.36%)	0% <0%> (ø)
... and 6 more

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[hengyunabc]

@CrazyHZM 这个 dubbo.attachments 是什么用途的？没看到有取出使用的地方。是从 RmiRemoteInvocation 里来的？