blip-0037: Bolt 12 encoded wallet `node_id`

[t-bast]

Bolt 12 introduces a new sciddir_or_pubkey field that can be used to identify the introduction node of a blinded path. This is primarily used to reduce the size of encoded offers, but this encoding trick can be extended to carry more information about the node it's referencing. We introduce an additional prefix to this field to tag node_ids that belong to mobile wallets.

When relaying a payment or an onion message, wallet providers usually want to offer additional features to mobile wallets. Encoding the information that the next node is a mobile wallet directly inside the onion payload makes it easy for the relaying node to efficiently insert additional steps before relaying (such as trying to wake-up the wallet if it is offline) without negatively impacting relay to normal nodes.