Consider adding a field for application-level metadata

[pirapira]

So far we have had the assumption that any non-protocol 3rd party data should be sent out of band and not be piggybacked to the actual raiden messages.

This issue keeps track of deciding

• wheather to add application-level metadata hash into Raiden messages
• and if yes, how
  • an additional field in Raiden messages?
  • make payment_identifier bigger than the current size (64bit)?

This issue is a spawn-out of a discussion.

[BOR4]

I am specing out a simple project on top of Raiden and this feature would help me in implementation a lot,

[loredanacirstea]

I am in favor of:

• adding application-level metadata hash into Raiden messages
• not using the payment_identifier, because it is currently used for other purposes in the client and 256bit might still be too small (from what I tested)
• we can have an additional field in Raiden messages, named additional_hash or metadata_hash (as proposed by @LefterisJP in Should offchain messages contain additional_hash? #3083 (comment)), that will be included in the integrity_hash (Rename additional_hash into message_hash or integrity_hash #3095)

[hackaugusto]

This is a duplicate of #1202