Support getting backup root path in plugin

[JohnNiang]

What type of PR is this?

/kind feature
/area core
/milestone 2.9.x

What this PR does / why we need it:

We already support backup and restore feature in Halo 2.8.0, but we cannot obtain backup files through regular channels in the plugin. For example, we want to upload backup files to OSS in the plugin.

This PR is aimed at solving this problem.

Does this PR introduce a user-facing change?

支持在插件中获取备份文件根目录。

[codecov]

Codecov Report

Merging #4422 (558d0ac) into main (905e867) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #4422      +/-   ##
============================================
+ Coverage     60.99%   61.01%   +0.02%     
- Complexity     2507     2509       +2     
============================================
  Files           370      371       +1     
  Lines         12910    12917       +7     
  Branches        928      928              
============================================
+ Hits           7874     7881       +7     
  Misses         4581     4581              
  Partials        455      455              

Files Changed	Coverage Δ
...va/run/halo/app/infra/DefaultBackupRootGetter.java	100.00% <100.00%> (ø)
.../halo/app/migration/impl/MigrationServiceImpl.java	84.55% <100.00%> (+0.12%)	⬆️
...alo/app/plugin/SharedApplicationContextHolder.java	96.55% <100.00%> (+0.25%)	⬆️

[guqing]

/approve

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing, ruibaby

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