Add a staking lock extension function allowing stakers to extend their lock period before it expires - #823

Summary

Adds stake_extend(key_id: BytesN<32>, additional_ledgers: u32), allowing a wallet with an active stake to push back its unlock_ledger and continue earning rewards without unstaking and re-staking. Introduces StakePosition storage keyed by wallet and BytesN<32> key ID, validates that the caller has an active stake on the given key (panicking with the new NoStakeFound error otherwise), enforces that the resulting unlock_ledger is no more than 180 days out from the current ledger (panicking with the new LockTooLong error otherwise), bumps the TTL on the staking_positions entry after a valid extension, and emits a stake_extended event carrying wallet, key ID, old unlock_ledger, and new unlock_ledger. Takes an explicit wallet argument rather than relying on an invoker lookup, since this Soroban SDK does not expose Env::invoker(). Changes are in lib.rs and events.rs.

Closes #800

Changes

• Added wallet/key-based StakePosition storage.
• Added stake_extend(wallet, key_id, additional_ledgers).
• Enforces a maximum unlock time of 180 days from the current ledger.
• Returns NoStakeFound when no active stake exists.
• Returns LockTooLong when the limit is exceeded.
• Updates the unlock ledger and bumps persistent storage TTL.
• Emits stake_extended with wallet, key ID, and old/new unlock ledgers.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item: none provided.
• Added tests for valid extension, NoStakeFound, LockTooLong, event emission, and TTL bump.
• Reviewed storage changes: the new StakingPosition key is additive; no existing keys are modified.
• Added a new stake_extended event with stable topic and payload field ordering.
• Updated documentation for the new public interface and storage behavior.
• Scope is limited to staking extension behavior.