[jdjioe5-cpu]

Summary

Resolves #2780.

Previously, \sign_soroban_authorizations\ unconditionally fetched the latest ledger sequence from RPC via \client.get_latest_ledger().await?.sequence\ and hardcoded \latest_ledger + 60. This bound the authorization signature expiration window solely to remote network state, making authorization validity unpredictable for offline signing, test harnesses, and client-side automation.

This PR exposes --signature-expiration-ledger\ (and \STELLAR_SIGNATURE_EXPIRATION_LEDGER):

1. CLI Flag & Env Var: Adds \signature_expiration_ledger: Option\ to \sign_with::Args.
2. Explicit Override: When provided, \sign_soroban_authorizations\ directly uses the specified expiration ledger without an unnecessary \get_latest_ledger\ network round-trip.
3. Fallback: When omitted, preserves existing behavior (\latest_ledger + 60).
4. Docs & Tests: Documents option in \SKILL.md\ and adds unit tests covering default, CLI argument, and environment variable resolution.

Verification

• \cargo test --package soroban-cli --lib config::sign_with::tests: Passes.
• Tested explicit flag override and fallback behavior.