FFI/GO: Add IAM authentication support with automatic token refresh

[affonsov]

Add IAM Authentication Support for Go Client

Overview

This PR implements IAM authentication support for the Go client, enabling secure connections to AWS ElastiCache and MemoryDB clusters using IAM credentials instead of static passwords.

Changes

Core Implementation

• IamAuthConfig: New configuration struct for IAM authentication

  • Supports both ElastiCache and MemoryDB service types
  • Configurable token refresh interval (default: 300 seconds)
  • Builder pattern with WithRefreshIntervalSeconds() method

• ServerCredentials Enhancement: Extended to support two authentication modes

  • Password-based authentication (existing)
  • IAM authentication (new)
  • Modes are mutually exclusive
  • Added IsIamAuth() helper method

• RefreshIamToken Method: Manual token refresh capability

  • Available on both standalone and cluster clients
  • Returns "OK" on success
  • Only works with IAM-authenticated clients

FFI Layer

• Added refresh_iam_token FFI binding in ffi/src/lib.rs
• Proper memory management with Arc reference counting
• Comprehensive safety documentation

Issue link

This Pull Request is linked to issue (URL): [REPLACE ME]

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one issue.
• Commit message has a detailed description of what changed and why.
• Tests are added or updated.
• CHANGELOG.md and documentation files are updated.
• Destination branch is correct - main or release
• Create merge commit if merging release branch into main, squash otherwise.