Create a method to return ad campaigns

[nima-karimi]

This issue is part of #678 for integrating with the Multichannel Marketing Dashboard released with WooCommerce 7.4.

The new marketing dashboard has a section dedicated to managing the marketing campaigns the user has created for their store. In the MVP version, they can only view their campaigns on that page. We need to prepare and return the following data about the campaigns to WooCommerce:

• The unique identifier of the campaign
• The name of the campaign
• The URL where the user can edit/view the details of the campaign
• The cost of the campaign (This can mean the spending budget associated with the campaign)
• The type of the campaign (Currently, Pinterest has an "Automatic" and "Manual" campaign type when creating a new ad campaign, but we should confirm if these two have any major differences aside from the way they are created). Note that the Pinterest's Get campaign API documentation indicates there is only one type, which is "campaign".

Since the extension does not store any data for the ad campaigns locally, we need to implement connections to the API to retrieve the list of the ad campaigns (ideally with a caching mechanism to avoid repeated requests to the API). Pinterest provides the data we need about the ads through its adsreporting APIs:

https://developers.pinterest.com/docs/redoc/adsreporting/#tag/campaigns

The list of campaigns can be obtained from this API:

https://developers.pinterest.com/docs/redoc/adsreporting/#operation/ads_v3_get_advertiser_campaigns_handler_GET

The advertiser ID can be obtained from the extension settings with the key tracking_advertiser:

$advertiser_id = Pinterest_For_Woocommerce()::get_setting( 'tracking_advertiser' );

[nima-karimi]

This issue has been blocked for a long time because of issues with the scope granted by Pinterest API auth (pcTzPy-vB-p2), so I'm moving it to the backlog.