feat: customize mortality

[sinzii]

This PR allows users to customize transaction mortality via a new mortality option in PayloadOptions — supporting immortal transactions and custom mortal periods

// Immortal transaction (never expires)
await client.tx.balances
  .transferKeepAlive(bob.address, amount)
  .signAndSend(alice, { mortality: { type: 'Immortal' } });

// Custom mortal period
await client.tx.balances
  .transferKeepAlive(bob.address, amount)
  .signAndSend(alice, { mortality: { type: 'Mortal', period: 128 } });

Huge thanks @antlerminator for reporting issues and suggesting this new feature!