Payload decoding in the browser wallet of the CIS3 standard

[DOBEN]

Task description
If a smart contract implements the CIS3 standard (sponsored transactions). The sponsored transaction message will be sent to the Concordium browser wallet to be signed by the user. Currently, when providing the schema to the browser wallet, it can decode the object except for the payload field which is an array of bytes.

Feature to be added to the browser wallet:
The browser wallet can also decode the payload field by looking up the parameter schema of the entry_point_name and using this schema to decode the payload. The payload is the input parameter that is sent to the entry_point_name.

CIS3 standard:
Concordium/concordium-update-proposals#41

[limemloh]

Since the current approach is to use signMessage function provided by the wallet allowing for signing an arbitrary message, we don't really know what we are signing, and we can't really deserialize the message and look whether it is for CIS-3 reliably.

1. A solution could be to extend the wallet API with signCis3Message which is tailored for this
2. Another could be to extend the JS-SDK with tools for constructing a CIS-3 schema type for this message, which then can be used for signMessage.

[abizjak]

Concordium/concordium-dapp-examples#77

[abizjak]

We agreed to go with solution 1 suggested above.