[ISSUE #811]Nacos support application discover

[haoyann]

The branch of 0.2.0 is master-0.2.0. If your PR is to solve the problem of 0.2.0, please submit the PR to it.
Please do not create a Pull Request without creating an issue first.

What is the purpose of the change

Fro #811

Brief changelog

XX

Verifying this change

XXXX

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [ISSUE #123] Fix UnknownException when host config not exist. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist.
• Run `mvn clean compile --batch-mode -DskipTests=false -Dcheckstyle.skip=false -Drat.skip=false -Dmaven.javadoc.skip=true to make sure basic checks pass.

[codecov-commenter]

Codecov Report

Merging #812 (87a5d82) into develop (f5954a7) will decrease coverage by 0.16%.
The diff coverage is 12.50%.

@@              Coverage Diff              @@
##             develop     #812      +/-   ##
=============================================
- Coverage      20.64%   20.48%   -0.17%     
- Complexity       413      415       +2     
=============================================
  Files            143      144       +1     
  Lines           4689     4755      +66     
  Branches         712      722      +10     
=============================================
+ Hits             968      974       +6     
- Misses          3645     3704      +59     
- Partials          76       77       +1     

Flag	Coverage Δ
unit	20.48% <12.50%> (-0.17%)	⬇️

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Impacted Files	Coverage Δ
.../org/apache/dubbo/admin/common/util/SyncUtils.java	0.00% <0.00%> (ø)
...min/registry/mapping/impl/NacosServiceMapping.java	6.89% <6.89%> (ø)
...apache/dubbo/admin/service/RegistryServerSync.java	75.34% <62.50%> (-1.47%)	⬇️

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