[5.1] Convert mod_custom to service provider

[joomlaweby]

Summary of Changes

Converts the custom administrator module to service provider.

Testing Instructions

1. Make sure you have administrator module mod_custom published and have some content in WYSIWYG
2. You should see module output

Actual result BEFORE applying this Pull Request

Module works as expected

Expected result AFTER applying this Pull Request

Module works same way as before applying pull request

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:

• No documentation changes for manual.joomla.org needed

[ghost]

I have tested this item ✅ successfully on f870f6c

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[viocassel]

I have tested this item ✅ successfully on f870f6c

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[richard67]

RTC

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[LadySolveig]

Thank you @joomlaweby and also for testing @fgsw @viocassel