Multichannel Marketing Backend MVP: API

[nima-karimi]

This issue is part of the epic #34548. It's also dependent on #34549.

As part of the multichannel marketing feature, we will implement APIs that the new marketing dashboard will use to retrieve and render the data provided by marketing channel extensions (through the method implemented in #34549).

The front end will use the API to interact with the marketing channels. In the MVP version, it will only be used to fetch the channels data and display them on the dashboard; however, in the future, the API will allow the creation of marketing campaigns too.

This API will be implemented under the Automattic\WooCommerce\Admin\API namespace. An API is already implemented for the marketing page under that namespace within the MarketingOverview class. However, since we will be using a feature toggle, it would be best to implement the new API under a new class and keep the current one intact. Later we can deprecate the MarketingOverview class and ultimately delete it.

GET marketing/channels

This endpoint utilizes the MarketingChannels service to fetch and return the list of active marketing channels. It will return an array of serialized MarketingChannelInterface objects; however, the campaign data need to be redacted because the front-end displays the aggregated campaign data (see endpoint below), not the campaigns grouped by channel.

GET marketing/campaigns

This endpoint returns the aggregated list of marketing campaigns for all channels. It will return an array of serialized MarketingCampaign objects. The ID/slug of the marketing channel should also be specified for each campaign and included in the data returned.