prefer jax-rs annotation on impl and compatible with old version

[Jeff-Lv]

As Luo's advice,

The jax-rs annoations should be preferred to implemation class as the reasons below

1. compatible to old dubbo version
2. Dubbo REST provider can be used without using of Dubbo Conumser for REST (User may access the rest API from JS client directly).
3. Interface should be kept clean without the pullation of REST annoatation.

[codecov-io]

Codecov Report

Merging #3224 into 2.7.0-release will increase coverage by 0.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##             2.7.0-release   #3224      +/-   ##
==================================================
+ Coverage            63.79%   63.8%   +0.01%     
  Complexity              75      75              
==================================================
  Files                  651     651              
  Lines                28311   28311              
  Branches              4792    4792              
==================================================
+ Hits                 18061   18064       +3     
+ Misses                7980    7976       -4     
- Partials              2270    2271       +1

Impacted Files	Coverage Δ	Complexity Δ
...g/apache/dubbo/rpc/protocol/rest/RestProtocol.java	64.75% <100%> (ø)	0 <0> (ø)	⬇️
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	0% <0%> (ø)
.../remoting/transport/netty4/NettyClientHandler.java	75% <0%> (-11.12%)	0% <0%> (ø)
...che/dubbo/remoting/transport/mina/MinaChannel.java	43.42% <0%> (-10.53%)	0% <0%> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	44.79% <0%> (-3.13%)	0% <0%> (ø)
.../exchange/support/header/HeaderExchangeServer.java	59.82% <0%> (-0.86%)	0% <0%> (ø)
.../java/org/apache/dubbo/config/ReferenceConfig.java	59.19% <0%> (+0.36%)	0% <0%> (ø)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	72.48% <0%> (+0.67%)	0% <0%> (ø)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	80.82% <0%> (+0.75%)	0% <0%> (ø)	⬇️
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	66.1% <0%> (+0.84%)	0% <0%> (ø)	⬇️
... and 8 more

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[chickenlj]

LGTM