feat: add Halo version variable to theme model

[guqing]

What type of PR is this?

/kind feature
/milestone 2.20.x
/area theme

What this PR does / why we need it:

主题支持通过 ${site.version} 得到 Halo 版本号

Which issue(s) this PR fixes:

Fixes #6676

Does this PR introduce a user-facing change?

主题支持通过 `${site.version}` 得到 Halo 版本号

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.09%. Comparing base (6cd8dc8) to head (1a5251f).
Report is 76 commits behind head on main.

Files with missing lines	Patch %	Lines
...a/run/halo/app/theme/finders/vo/SiteSettingVo.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6677      +/-   ##
============================================
- Coverage     58.18%   58.09%   -0.09%     
- Complexity     3774     3937     +163     
============================================
  Files           651      675      +24     
  Lines         22125    23116     +991     
  Branches       1538     1578      +40     
============================================
+ Hits          12873    13430     +557     
- Misses         8641     9065     +424     
- Partials        611      621      +10     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[JohnNiang]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

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