Service name mapping support

[flycash]

What this PR does:

Which issue(s) this PR fixes:

Fixes #419

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #453 into feature/dubbo-2.7.5 will increase coverage by 0.29%.
The diff coverage is 66.66%.

@@                   Coverage Diff                   @@
##           feature/dubbo-2.7.5     #453      +/-   ##
=======================================================
+ Coverage                66.93%   67.22%   +0.29%     
=======================================================
  Files                      176      177       +1     
  Lines                     9342     9374      +32     
=======================================================
+ Hits                      6253     6302      +49     
+ Misses                    2476     2458      -18     
- Partials                   613      614       +1

Impacted Files	Coverage Δ
config_center/apollo/impl.go	82.71% <0%> (-2.1%)	⬇️
config_center/nacos/impl.go	71.08% <0%> (-1.76%)	⬇️
config_center/zookeeper/impl.go	47.5% <58.33%> (+3.46%)	⬆️
metadata/dynamic_mapping/service_name_mapping.go	88.23% <88.23%> (ø)
cluster/cluster_impl/failback_cluster_invoker.go	78.49% <0%> (-2.16%)	⬇️
protocol/dubbo/readwriter.go	70.73% <0%> (+2.43%)	⬆️
remoting/kubernetes/listener.go	65.42% <0%> (+2.8%)	⬆️
protocol/dubbo/pool.go	81.27% <0%> (+4.56%)	⬆️
... and 2 more

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[AlexStocks]

hi, guy, is it a good idea naming a package like " dynamic_mapping" or "in_memory"? I do not think so.

[hxmhlt]

hi, guy, is it a good idea naming a package like " dynamic_mapping" or "in_memory"? I do not think so.

Or dynamicmapping and inmemory without _？
I found other project is named the package like this.

[flycash]

I don't know what is the common practice in Golang. Following the common practice is a good idea.

[AlexStocks]

I don't know what is the common practice in Golang. Following the common practice is a good idea.

refers to the package name in github.com/golang/go.

[zouyx]

what about mapping/dynamic and mapping/memory

[pantianying]

LGTM

[hxmhlt]

LGTM

[fangyincheng]

LGTM