Action: check `filecoinpin.contact/cid/$cid` before showing IPFS Mainnet retrieval URLs

[BigLep]

Done Criteria

The example GitHub Action doesn't show the closing summary with IPFS Mainnet retrieval URLs until it confirms that filecoinpin.contact/cid/$cid is non-empty.

Why Important

1. We don't want to set the expectation that IPFS Mainnet retrieval works by showing URLs if we can know ourselves that it won't if there is no record in filecoinpin.contact.
2. Prematurely accessing IPFS Mainnet URLs makes the situation worse because of the negative caching in cid.contact.

Notes

1. This filecoinpin.contact logic already exists in filecoin-pin-website. Ideally we pull it in as a helper function to filecoin-pin and then the example action and filecoin-pin-website can use it.
2. While the helper function today may be tied to filecoinpin.contact polling, ideally the helper is named more abractly like validateIpniAdvertisment(String cid)