v1.7.1 - Add AllAccountBalances API and Fix preprocess-operations URL Naming Consistency

Adding two enhancements to the Rosetta API specification:

• AllAccountBalances API - New POST /account/all-balances endpoint to retrieve all sub-account balances in a single request
• URL Naming Fix - Changed /construction/preprocess_operations to /construction/preprocess-operations for kebab-case consistency

What's Changed

• feat: All account balances by @jacquescaocb in #128

Full Changelog: v1.6.1...v1.7.1