Add "long" payment reference calculator function

[MantisClone]

Problem(s)

The RN SDK provides a PaymentReferenceCalculator calculates the "short" payment reference but not the "long" payment reference.

Also, the short and long payment Reference aren't in the docs.

Proposed Solution

• Add a function to the PaymentReferenceCalculator that calculates the "long" payment reference.
• Add documentation about what is short and long payment reference with links to the calcualtor.

Details

• The payment reference input to transferFromWithReferenceAndFee() is 8-byte "short" payment reference (see attached image)
• The payment reference output from the TransferWithReferenceAndFee event is the "long" keccak256 hash of the 8-byte short payment reference (see attached image)
• The payments-subgraph indexes the "long" keccak256 hash of the 8-byte short payment reference
• The RN SDK calculates the "long" hash when querying the payments subgraph <-- Use this to create the new function in PaymentReferenceCalculator.

Additional Detail

• If someone gets the "long" payment reference from the payments-subgraph, there's no way to figure out the corresponding "short" payment reference. I think this is okay 🙆‍♀️

Reference

• paymentReference input spec
• paymentReference output spec
• paymentReference input to transferFromWithReferenceAndFee()
• paymentReference output from TransferWithReferenceAndFee event

Screenshots