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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarqubecloud]

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [JohnNiang]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment