Implement get_ledger_sequence

[bbyalcinkaya]

In the Stellar blockchain, what is typically referred to as a "block" in other blockchains is known as a "ledger." The ledger sequence number represents the current block number.

This PR includes the following changes:

1. Add configuration cell <ledgerSequenceNumber>
2. Implement get_ledger_sequence host function
   • Allows contracts to access <ledgerSequenceNumber>.
3. Implement Kasmer cheatcode kasmer_set_ledger_sequence:
   • Enables Kasmer test contracts to set <ledgerSequenceNumber>.
4. Add an integration test that exercises these functions.