feat(fresh_dio): implement token refresh locking mechanism

[ablause]

Fixes #126

Problem

When multiple HTTP requests receive 401 responses simultaneously, each request independently triggers refreshToken, resulting in multiple concurrent refresh operations instead of a single one. While QueuedInterceptor queues incoming requests, it doesn't prevent multiple requests that are already in the error/response handlers from independently calling refreshToken. This occurs because:

1. Multiple requests receive 401 responses nearly simultaneously
2. Each enters onError/onResponse and calls _tryRefresh()
3. All requests see that no refresh is in progress and each initiates a new refresh
4. Result: refreshToken is called N times for N concurrent failed requests

Solution

This PR implements a synchronization mechanism using the synchronized package:

1. Lock-based synchronization: Uses Lock from synchronized to ensure only one refresh operation executes at a time
2. Token caching: Stores a reference to the last refreshed token in _latestRefreshedToken
3. Identity check: Before refreshing, verifies if the current token is identical to the cached token using identical(). If yes, reuses it without refreshing

[NapasPayu]

@felangel could you please take a look?