[5.2][Events] Use event classes for Finder plugins

[Fedik]

Summary of Changes

Update finder plugins to use SubscriberInterface and an event classes.

Testing Instructions

Run finder indexer, edit the content.
Check the indexer works, and changed content also indexed.

Actual result BEFORE applying this Pull Request

Works

Expected result AFTER applying this Pull Request

Works

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: New Event classes Manual#177
• No documentation changes for manual.joomla.org needed

[exlemor]

I have tested this item ✅ successfully on 2912da3

I tested this successfully.

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[ghost]

I have tested this item ✅ successfully on 2912da3

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/43607.}

[richard67]

RTC

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[Hackwar]

Thank you for your contribution @Fedik!