HACK week. Part of #15372

Description

This PR registers a new Keychain merchantAIProviderKey value for storing the associated merchant's AI API key, to be used in AI Settings when Jetpack AI is not used by default.

Testing information

• In AISettingsView .onAppear method, add:

debugPrint("\(viewModel.keychain)")
debugPrint("\(viewModel.keychain[WooConstants.merchantAIProviderKey])")

• Enable allowMerchantAIAPIKey flag
• Go to Menu > AI Settings.
• Play around with the API field. Save, clear, update, get out, reload the view, etc, ..
• Note how the key is stored/cleared/updated adequately
• You'll also see the key and value printed in the console:

$ [\"key\": \"merchantAIProviderKey\", [ ... ]
$ "Optional(\"This is a test\")"