[CAP-90] Custom signature payload for account contracts

[dmkozh]

This is a discussion thread for CAP-90: Custom signature payload for account contracts.

This CAP aims at closing the gap in the Soroban custom account design: it allows every custom account to authenticate an arbitrary, custom payload in addition to the fixed, protocol-defined context that Soroban host currently provides. That increases the flexibility of custom accounts and allows implementation of new functionality from custom nonce policies to policy rules in modular accounts that utilize delegation. The latter custom policy use case was the main driver for this CAP.

[JoseCToscano]

Supportive of this. We're working on spending policies for agents paying via x402 on Stellar, and committing to a policy rule id in the custom payload, plus account-managed nonces, would make enforcement in __check_auth much cleaner.

One note: the x402 Stellar exact scheme currently only accepts ADDRESS/ADDRESS_V2 credentials and disallows delegates, so the x402 spec would need a small update to support V3.