Add polaris extensions support

[andrewshan]

What is the purpose of the change

Add the polaris extensions to make dubbo2.7.x access polaris

Brief changelog

#91

Verifying this change

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[codecov-commenter]

Codecov Report

Merging #184 (8fc5151) into master (9312264) will increase coverage by 4.41%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #184      +/-   ##
============================================
+ Coverage     34.27%   38.68%   +4.41%     
+ Complexity      516      488      -28     
============================================
  Files           151      140      -11     
  Lines          5313     4513     -800     
  Branches        657      570      -87     
============================================
- Hits           1821     1746      -75     
+ Misses         3292     2569     -723     
+ Partials        200      198       -2     

Impacted Files	Coverage Δ
...che/dubbo/remoting/transport/mina/MinaChannel.java	35.52% <0.00%> (-10.53%)	⬇️
...org/apache/dubbo/registry/redis/RedisRegistry.java	52.05% <0.00%> (-2.63%)	⬇️
...he/dubbo/rpc/protocol/hessian/HessianProtocol.java	77.64% <0.00%> (-2.36%)	⬇️
...g/apache/dubbo/registry/consul/ConsulRegistry.java
.../support/etcd/EtcdDynamicConfigurationFactory.java
.../dubbo/registry/consul/ConsulServiceDiscovery.java
.../apache/dubbo/remoting/etcd/jetcd/RetryNTimes.java
.../dubbo/metadata/store/etcd/EtcdMetadataReport.java
...ubbo/remoting/etcd/support/AbstractEtcdClient.java
...java/org/apache/dubbo/remoting/etcd/Constants.java
... and 20 more

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[AlbumenJ]

Please enhance uts. Ohters LGTM