[4.3] Migrate quickicon downloadkey plugin to services

[heelc29]

Summary of Changes

Migrate the quickicon downloadkey plugin to service providers and use SubscriberInterface.

Testing Instructions

• Install an extension with providing a download key
• Check if quickicons are still working

Actual result BEFORE applying this Pull Request

Downloadkey quickicon is working

Expected result AFTER applying this Pull Request

Downloadkey quickicon is working (and the other quickicons are still displayed)

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

[laoneo]

Guess this here should be lower case as well https://github.com/joomla/joomla-cms/pull/39168/files#diff-6886871adeab8130a00db34ba210255b45c247430a320dc497c4ffd88c06ee13R5

[laoneo]

I have tested this item ✅ successfully on 3ad401b

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[heelc29]

Guess this here should be lower case as well

Done

[Abernyte-Git]

I have tested this item ✅ successfully on 254dcee

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[alikon]

RTC

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[obuisard]

Great work @heelc29. Thank you!

[heelc29]

Thanks