Retrieves details about a specific payment plan by its ID.
const plan = await payments.getPlan(planId)
{ "planId": "string", "metadata": { "name": "string", "description": "string" }, "price": { "priceType": "FIXED_PRICE", "tokenAddress": "string", "amounts": ["number"], "receivers": ["string"] }, "credits": { "creditsType": "FIXED", "amount": "number", "minAmount": "number", "maxAmount": "number", "durationOfThePlan": "number" }, "nftAddress": "string?" }