Integrate with Multichannel Marketing

The new multichannel marketing dashboard is being implemented in WooCommerce (https://github.com/woocommerce/woocommerce/issues/34548) and will be released soon. For this extension to be recognized as a marketing channel and be displayed on that dashboard, we need to implement an integration with the interfaces/classes introduced in the WooCommerce core.

Details on implementing this integration are currently available here: pe2C5g-zb-p2

### Tasks
- [ ] https://github.com/woocommerce/pinterest-for-woocommerce/issues/697
- [ ] https://github.com/woocommerce/pinterest-for-woocommerce/issues/699
- [ ] Implement other methods in MarketingChannelInterface and register as a channel

In short, we need to implement the MarketingChannelInterface in the code base and register an instance using the MarketingChannels class. The implementation of MarketingChannelInterface will return information about the Pinterest marketing channel, such as its name, descriptions, setup_url, icon, etc., and WooCommerce will display this information to the user on the new multichannel marketing dashboard.

Most of the data returned by the MarketingChannelInterface methods are straightforward to gather and implement. However, returning the following data might be more complicated:

`get_product_listings_status`

This will return whether the product listings are synchronized with Pinterest or the synch is in progress.

The ProductSync::is_product_sync_enabled method can be used to check if product syncing functionality is enabled. If it's not enabled, we can return the MarketingChannelInterface::PRODUCT_LISTINGS_NOT_APPLICABLE constant.

If product syncing is enabled, we would need to return either of these constants as the status:

MarketingChannelInterface::PRODUCT_LISTINGS_SYNC_IN_PROGRESS - If sync is still in progress.
MarketingChannelInterface::PRODUCT_LISTINGS_SYNC_FAILED - In case there are errors when syncing the products.
MarketingChannelInterface::PRODUCT_LISTINGS_SYNCED - If syncing the products has been successful.

The FeedState::get_feed_state method seems to be a good starting point for obtaining this information. However, the data structure returned by this method is custom-tailored to be used by the front end.

`get_errors_count`

This should return the number of errors/issues reported by Pinterest for the products and/or the account. This data can be obtained from the FeedState::get_feed_state method as well.

We can cache this data locally to prevent querying the Pinterest API whenever WooCommerce loads this information.

`get_supported_campaign_types`

This will define what types of marketing campaigns the Pinterest extension supports. It should return an array of MarketingCampaignType objects that define the following properties:

Property Name	Property Type	Description
`id`	`string`	A unique identifier for the campaign type. For example pinterest-ads
`name`	`string`	Name of the marketing campaign type.
`description`	`string`	Description of the marketing campaign type.
`create_url`	`string`	The URL to the create campaign page.
`icon_url`	`string`	The URL to an image/icon for the campaign type.
`channel`	`MarketingChannelInterface`	Specifies the marketing channel that his campaign type belongs to. If you define the types from within the class implementing the `MarketingChannelInterface`, then the channel should be set to `$this`.

Currently, there are two ad campaign types for Pinterest: Automatic and Manual. The user will be redirected to the Pinterest "Campaign creation page (https://ads.pinterest.com/advertiser/{ADS_ACCOUNT_ID}/ads/campaign_mode/), and they are presented with these two options to create campaigns:

`get_campaigns`

The list of all ad campaigns must be mapped to an array of MarketingCampaign objects and returned by this method. Each MarketingCampaign object has the following properties:

Property Name	Property Type	Description
`id`	`string`	The unique identifier of the marketing campaign.
`title`	`string`	The title of the campaign.
`manage_url`	`string`	The URL to the channel’s campaign management page.
`cost`	`Price`	The cost of the campaign. This should be an object of the type Price.
`type`	`MarketingCampaignType`	The type of this marketing campaign. It should match one of the supported types returned by the `MarketingChannelInterface::get_supported_campaign_types`

Since the extension does not store any of the ad campaigns locally, we need to implement connections to the API to retrieve the list of the ad campaigns and then have a caching mechanism for the campaigns to store them locally. Pinterest provides the data we need about the ads through its APIs.

From pe2C5g-8v-p2#pinterest-for-woocommerce-github-wccom:

The campaigns themselves appear to include a name and a daily budget, but geo targeting seems to be less common (N/A could be displayed in the respective column (of) the MCM hub).

woocommerce / pinterest-for-woocommerce