feat: add system info getter for plugin

[guqing]

What type of PR is this?

/kind improvement
/area core
/milestone 2.20.x

What this PR does / why we need it:

为插件提供 SystemInfoGetter 用于获取站点基本信息

站点标题描述和系统版本这些信息对于插件来说是很有必要的避免插件要直接查询 system ConfigMap 来获取，如 RSS 和 通知器扩展等插件都会需要用到

{
  "title" : "guqing's blog",
  "subtitle" : "副标题",
  "logo" : "/upload/myavatar.png",
  "favicon" : "/upload/myavatar.png",
  "url" : "http://localhost:8090",
  "version" : {
    "majorVersion" : 2,
    "minorVersion" : 20,
    "normalVersion" : "2.20.10",
    "preRelease" : true,
    "publicApiStable" : true,
    "patchVersion" : 10,
    "preReleaseVersion" : "SNAPSHOT",
    "buildMetadata" : "",
    "stable" : false
  },
  "seo" : {
    "blockSpiders" : false,
    "keywords" : "keyword1,keyword2",
    "description" : "站点描述"
  },
  "locale" : "zh_CN_#Hans",
  "timeZone" : "Asia/Shanghai",
  "activatedThemeName" : "theme-earth"
}

Does this PR introduce a user-facing change?

开发者相关：为插件提供 SystemInfoGetter 用于获取站点基本信息

[codecov]

Codecov Report

Attention: Patch coverage is 8.16327% with 45 lines in your changes missing coverage. Please review.

Project coverage is 57.19%. Comparing base (eff73dc) to head (1d6e7d4).
Report is 74 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/run/halo/app/infra/SystemInfoGetterImpl.java	3.22%	30 Missing ⚠️
...i/src/main/java/run/halo/app/infra/SystemInfo.java	0.00%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7103      +/-   ##
============================================
+ Coverage     56.99%   57.19%   +0.19%     
- Complexity     3999     4047      +48     
============================================
  Files           714      719       +5     
  Lines         24110    24348     +238     
  Branches       1585     1598      +13     
============================================
+ Hits          13742    13925     +183     
- Misses         9756     9805      +49     
- Partials        612      618       +6     

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

[sonarqubecloud]

Quality Gate passed

Issues
2 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 SonarQube Cloud

[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