Access the api through nevermined

[r-marques]

Describe how to access the api through the nevermined proxy

1. instantiate the sdk
2. login to nevermined
3. buy the subscription
4. check the buyer is a holder of the subscription
5. the buyer should be able to retrieve the Bearer Token to access the api
6. show how to access the service using curl

Reference: https://github.com/nevermined-io/sdk-js/blob/main/integration/external/Services_NFT1155.e2e.test.ts