provider can get attachment in ctx

[pantianying]

What this PR does:

before this , consumer can send attachment from context
after this
Provider get attachment in context

[codecov-io]

Codecov Report

Merging #508 into develop will increase coverage by 0.04%.
The diff coverage is 25.00%.

@@             Coverage Diff             @@
##           develop     #508      +/-   ##
===========================================
+ Coverage    66.38%   66.43%   +0.04%     
===========================================
  Files          184      184              
  Lines         9717     9720       +3     
===========================================
+ Hits          6451     6457       +6     
+ Misses        2626     2625       -1     
+ Partials       640      638       -2     

Impacted Files	Coverage Δ
common/proxy/proxy_factory/default.go	20.63% <0.00%> (-0.34%)	⬇️
common/proxy/proxy.go	91.26% <33.33%> (-1.81%)	⬇️
cluster/cluster_impl/base_cluster_invoker.go	62.31% <0.00%> (-10.15%)	⬇️
remoting/kubernetes/listener.go	56.07% <0.00%> (ø)
remoting/kubernetes/watch.go	79.24% <0.00%> (+0.94%)	⬆️
config_center/nacos/client.go	57.26% <0.00%> (+1.70%)	⬆️
cluster/cluster_impl/failback_cluster_invoker.go	80.64% <0.00%> (+2.15%)	⬆️
config_center/nacos/facade.go	62.50% <0.00%> (+25.00%)	⬆️

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[flycash]

attachment本质上是key-value，如果只是把attachment里面的key-value拉平塞进去context，而不是直接塞整个attachment，这样是否可行？

[pantianying]

attachment本质上是key-value，如果只是把attachment里面的key-value拉平塞进去context，而不是直接塞整个attachment，这样是否可行？

这里主要是参照java的模式 java中是通过context.getAttachment()获取到一个map[string]string ,而且客户端发送attachment之前已经确定格式了 现在也不好改了

[cvictory]

晚点合并，等 #495 合并完成之后，再在纸上修改吧。

[Patrick0308]

dubbo-go/common/proxy/proxy_factory/default.go

Line 89 in c6be554

result.SetAttachments(invocation.Attachments())

This line of code should be modified, otherwise modified attachment will not be returned.

[zouyx]

Still failing. boss Pan

[pantianying]

dubbo-go/common/proxy/proxy_factory/default.go

Line 89 in c6be554

result.SetAttachments(invocation.Attachments())

This line of code should be modified, otherwise modified attachment will not be returned.

现在暂不考虑服务端修改的ctx返回，因为ctx的传值机制是新生成ctx对象，只能向下传递，暂时无法向上传递修改内容