Allow plugin to listen the event the plugin has started

[JohnNiang]

What type of PR is this?

/kind feature /area core /milestone 2.17.x

What this PR does / why we need it:

This PR add support for allowing plugin to listen the event that the plugin has started. Below is an example of listening the event in plugin:

    @EventListener
    void onPluginStartedEvent(PluginStartedEvent event) {
        // do something.
    }

See https://github.com/halo-dev/halo/issues/5339#issuecomment-2199220068 for more.

Which issue(s) this PR fixes:

Fixes https://github.com/halo-dev/halo/issues/5339#issuecomment-2199220068

Special notes for your reviewer:

1. Create a plugin, add the listener above and write some logs
2. Build and install the plugin
3. Start plugin and see the logs you wrote

Does this PR introduce a user-facing change?

支持在插件中监听已启动事件

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 7.69231% with 12 lines in your changes missing coverage. Please review.

Project coverage is 57.19%. Comparing base (bc10336) to head (70f5e17). Report is 44 commits behind head on main.

Files	Patch %	Lines
...in/java/run/halo/app/plugin/HaloPluginManager.java	9.09%	10 Missing :warning:
.../run/halo/app/plugin/event/PluginStartedEvent.java	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6234 +/- ## ============================================ + Coverage 54.51% 57.19% +2.68% - Complexity 3523 3719 +196 ============================================ Files 646 647 +1 Lines 21862 21974 +112 Branches 1528 1538 +10 ============================================ + Hits 11917 12567 +650 + Misses 9328 8798 -530 + Partials 617 609 -8 ```

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[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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