Add filter method to ledger, balance and transaction

[rafaelcarvalhocaetano]

Summary

Currently there's no way to filter ledgers, balances, or transactions using the SDK. The Blnk API supports filter endpoints that allow querying resources with various operators.

Proposed Solution

Add a Filter() method to the following services:

• LedgerService
• LedgerBalanceService
• TransactionService

The implementation should include:

• A Filter struct with Field, Operator, and Value fields
• A FilterRequest struct supporting pagination (Limit, Page), sorting (SortBy, SortOrder), and count inclusion (IncludeCount)
• Validation for filter operators (e.g., gt, gte, lt, lte require numeric/date values; like, ilike require strings)
• Comprehensive unit tests

API Endpoints

• POST /ledgers/filter
• POST /balances/filter
• POST /transactions/filter