[Secret NFTs] Decrypt and view

[ipapandinas]

Atomic helpers

• [x] getSgxEnclaves()* => return enclaves url
• [x] getSignature(keyring, data)** => u8aToHex(sender.sign(data)) [SAME AS THE ONE ON MINTING]
• [x] getSSSShares()***^
• [x] reconstructPrivateKey(): SSS shares combinaison
• [x] ipfsEncryptedFileGetter(): (get encrypt file + get public key)
• [x] decryptFile(key pairs)^
• [ ] saveFileOnDisk()

*Hardcoded first, in the future query chain storage to discover enclaves *data = nftId + accountPubKey + sssaShare **POST call with signature \^provide some information about the upload progress state*

[ipapandinas]

Cleanings in PR: #137

saveFileOnDisk is not implemented yet.