Add billing status info in the Settings UI

[budzanowski]

Changes proposed in this Pull Request:

This will be useful to merchants but I have quickly developed it to know if the advertiser_id I am using for testing has correct billing setup.

Screenshots:

Invalid billing status:

Valid billing status

Detailed test instructions:

1. Go to wp-admin -> Marketing -> Pinterest -> Connection
2. Observe billing status with Advertiser that has correct billing and with advertiser that has no billing status set
3. Make sure that the button takes you to the billing settings.

Additional details:

Changelog entry

Add - Billing status UI in Connection tab.

[budzanowski]

@ecgan than you for the review 👍🏻

Everything has been adjusted.