feat(contract): add schedule_ttl_bump callable by anyone to extend a schedule's persistent storage lifetime

[libby-coder]

Background

Once schedules are moved to persistent storage (see storage TTL issue), their TTL will need to be extended periodically or they expire. A bump_schedule_ttl(schedule_id) entry point would allow anyone (beneficiary, grantor, or a third-party keeper) to extend the TTL without modifying the schedule.

Proposed API

pub fn bump_schedule_ttl(env: Env, schedule_id: u64)

Calls env.storage().persistent().extend_ttl(key, min, max) for the schedule entry. Panics with "Schedule not found" for unknown IDs.