Refactor support for multiple AI providers

[InAnYan]

For every AI provider, these pieces of information should be saved:

• API key
• Chat model
• API base URL

We should refactor the code, so that it will be easier to add new AI provider in the future

[InAnYan]

More discussion was there #104

[koppor]

Note that #104 is about the "Api Keys". The API key handling needs to be separated from the other preferences - to enable late loading.

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We need something like "TrustStoreManager", which provides the API key at the latest point in time.

The apiKeys need also to be removed from org.jabref.logic.importer.ImporterPreferences#ImporterPreferences.