[HACK] AI API keys: Switch generative content remote based on ai source

[iamgabrielma]

HACK week. Part of #15372

Description

This PR adds a new implementation of the GenerativeContentRemoteProtocol via MerchantGenerativeContentRemote. At the moment uses OpenAI as the only source, but later we'll expand to Anthropic as well.

At the moment we just assume that when the feature flag is on, we'll always use the merchant's API key, and when the flag is off, we fallback to the default behavior (AI eligibility checks to use Jetpack as AI source, or none).

Steps to reproduce

Testing information

Screenshots

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[dangermattic]

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16142-cf1238c
	Version	23.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	cf1238c
	Installation URL	17dtg6bqj0i58

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[iamgabrielma]

Closing: pdfdoF-85K#comment-9421-p2