Issue #3387111: Allow alttext.ai to use the keys module

Alter the Service plugins to allow consistent use of the key module and to only use the credentials config, ensuring that the API key is only stored in config if the user explicitly selected the local config provider and is never stored in config if the key module is selected.

added 4 commits

• d72f63b6...ccb6f1ea - 3 commits from branch project:2.0.x
• c8331ca0 - Issue #3387111: Allow alttext.ai to use the keys module

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