Add generator meta into head

[JohnNiang]

What type of PR is this?

/kind feature
/area core
/milestone 2.11.x

What this PR does / why we need it:

Please see https://html.spec.whatwg.org/multipage/semantics.html#meta-generator for more.

This PR add the generator meta into head, so that we can know what sites are using Halo.

http localhost:8090/ | grep generator

<meta name="generator" content="Halo 2.11.0-SNAPSHOT"/>

If someone want to disable the generator meta, they can configure the property halo.theme.generator-meta-disabled to true.

Does this PR introduce a user-facing change?

添加 Generator 元数据标识

[ruibaby]

建议在配置文件提供一个开关。

[codecov]

Codecov Report

Merging #4821 (5ee0240) into main (caa4d44) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #4821      +/-   ##
============================================
+ Coverage     61.49%   61.53%   +0.03%     
- Complexity     2806     2811       +5     
============================================
  Files           405      406       +1     
  Lines         14807    14820      +13     
  Branches       1030     1030              
============================================
+ Hits           9106     9119      +13     
  Misses         5205     5205              
  Partials        496      496              

Files	Coverage Δ
...run/halo/app/infra/properties/ThemeProperties.java	71.42% <100.00%> (+4.76%)	⬆️
...in/java/run/halo/app/theme/ThemeConfiguration.java	50.00% <100.00%> (+1.61%)	⬆️
...halo/app/theme/dialect/GeneratorMetaProcessor.java	100.00% <100.00%> (ø)

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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 [guqing]

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