Policies: Make allowed recipients and amount fields optional

[alberto-crossmint]

Make limit and allowed_recipients optional in TokenTransferPolicy

Changes both fields to Option<T> to enable cleaner semantics:

• limit: None — no spending cap; no tracker created/maintained
• allowed_recipients: None — any recipient allowed
• allowed_recipients: Some([]) — explicit deny-all

Previously, empty/zero values were ambiguous. All other checks (token, expiration, recipients) still apply regardless of whether a limit is set.

Changes:

• TokenTransferPolicy struct updated in smart-account-interfaces
• on_add only initializes spend tracker when limit is Some
• check_auth skips tracker logic when limit is None
• Added tests for None-limit and Some([])-recipients behavior
• Updated README example