[ISSUE #760]Application discover support

[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

For #760

Brief changelog

1. Support application discover
2. Compatible 2.x version metadata serialization

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 #807 (36a98a7) into develop-for-dubbo-3.x (c32a0f9) will decrease coverage by 0.79%.
The diff coverage is 11.85%.

@@                     Coverage Diff                     @@
##             develop-for-dubbo-3.x     #807      +/-   ##
===========================================================
- Coverage                    21.37%   20.57%   -0.80%     
- Complexity                     394      412      +18     
===========================================================
  Files                          129      143      +14     
  Lines                         4309     4685     +376     
  Branches                       676      712      +36     
===========================================================
+ Hits                           921      964      +43     
- Misses                        3308     3645     +337     
+ Partials                        80       76       -4     

Flag	Coverage Δ
unit	20.57% <11.85%> (-0.80%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../org/apache/dubbo/admin/common/util/Constants.java	0.00% <ø> (ø)
...pache/dubbo/admin/common/util/ServiceTestUtil.java	0.00% <ø> (ø)
...che/dubbo/admin/common/util/ServiceTestV3Util.java	0.00% <0.00%> (ø)
.../org/apache/dubbo/admin/common/util/SyncUtils.java	0.00% <0.00%> (ø)
...ache/dubbo/admin/controller/ServiceController.java	10.20% <0.00%> (-1.43%)	⬇️
.../dubbo/admin/controller/ServiceTestController.java	11.42% <0.00%> (-5.24%)	⬇️
...ubbo/admin/model/domain/FullServiceDefinition.java	0.00% <0.00%> (ø)
...che/dubbo/admin/model/domain/MethodDefinition.java	0.00% <0.00%> (ø)
.../org/apache/dubbo/admin/model/domain/Provider.java	0.00% <0.00%> (ø)
...he/dubbo/admin/model/domain/ServiceDefinition.java	0.00% <0.00%> (ø)
... and 34 more

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