Fineract-Client-Feign usage for Integration Tests

Currently, Apache Fineract's integration testing suite relies heavily on low-level RestAssured API calls, which results in verbose and fragile tests burdened by raw HTTP boilerplate. As the codebase continues to scale, maintaining this infrastructure significantly bottlenecks developer velocity and complicates code reviews. This project will modernize the testing suite by migrating these tests to the type-safe, higher-level fineract-client-feign SDK. By enforcing this pattern, the project will instantly reduce boilerplate, guarantee compile-time type safety, and standardize how future integration tests are authored. The core deliverables for this initiative include: * Migrating 75 core common and utility helpers to invoke the API using fineract-client. * Converting 275+ standalone integration tests across every domain to use the new SDK-backed helpers. * Completely removing the RestAssured dependency from the integration-tests build configuration. * Creating comprehensive developer documentation outlining the SDK-first approach and authentication patterns for future contributors.