Add tracking for campaign count

[mikkamp]

Changes proposed in this Pull Request:

This PR adds tracking for campaign count and the ads customer ID.

Since the campaign count is retrieved from the Ads API the result is cached for 12 hours to reduce any delays. Since we also fetch a list of campaigns when viewing the dashboard, we use this opportunity to cache the count so when the tracker snapshot is sent it can use a cached value.

The cached values are cleared upon a disconnect, when a campaign is created, or if the transient expires.

Closes #2130

Detailed test instructions:

1. Test on a site with MC and Ads accounts onboarded
2. Use a code snippet to output a tracking snapshot to a page:

add_action(
	'admin_init',
	function() {
		if ( ! empty( $_GET['wctracker'] ) ) {
			$tracks = apply_filters( 'woocommerce_tracker_data', [] );
			echo '<pre>' . json_encode( $tracks, JSON_PRETTY_PRINT ) . '</pre>';
			exit;
		}
	}
);

3. Visit the page: https://domain.test/wp-admin/?wctracker=1 (swap for your admin domain)
4. Confirm the snapshot data has the correct values for ads_customer_id and ads_campaign_count
5. Disconnect the ads account and recheck the snapshot data
6. Confirm the values are both 0
7. Confirm that the transient _transient_gla_ads_campaign_count is cleared up (after the disconnect)
8. Reconnect and view the list of campaigns on the dashboard page
9. Check the DB (or transient manager) to confirm that the transient _transient_gla_ads_campaign_count has been populated

Changelog entry

• Tweak - Add tracking for campaign count.

[martynmjones]

Hey @mikkamp, thanks for your work to improve tracking.

Tested this branch and confirmed that the number of campaigns is cached and accurately reported when tracking is enabled so LGTM ✅