[CAP-73] Simplify the inteface for creating trustlines.

[dmkozh]

Also removed mentions of CAP-72 from most sections, as it seems like it's unlikely that we'll do CAP-72.

Without CAP-72 there is not much motivation to provide granular control control over trustline limits from Soroban or checking if a trustline exists in the first place. Thus just one simple function for creating trustlines when necessary is introduced instead of 2 functions we had before.

[Copilot]

Pull request overview

This PR updates CAP-0073 to simplify the Soroban interface for managing classic trustlines, focusing on a single allow_trust function and de-emphasizing CAP-72. It also clarifies how XLM SAC transfers create accounts and updates the design rationale accordingly.

Changes:

• Replace the previous change_trust and has_trust SAC functions with a single allow_trust(env: Env, address: Address) function that creates an unlimited trustline if it is missing.
• Clarify semantics for allow_trust, including no-op behavior for C-addresses, required authorization only on trustline creation, and use of i64::MAX as the trustline limit.
• Remove most references to CAP-72 and adjust the XLM transfer and design rationale sections to match the new, simplified trustline-creation behavior.

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