dio_essentials: Four Dio interceptors for deduplication, retries, token refresh and logging

Developers using the Dio HTTP client often hand-write the same four interceptors for retries, token refresh, logging and request deduplication, and these tend to share bugs. The pure Dart package dio_essentials provides all four, depending only on dio and working on the VM, Flutter and the web. Its deduplication merges identical in-flight requests into one network call, with an optional short-lived cache for GET and HEAD, while writes are never cached or replayed. The retry interceptor uses exponential backoff with full jitter, honours Retry-After headers, and retries writes only when they are marked idempotent. The token interceptor performs a single shared refresh on a 401 and replays waiting requests, and the logger redacts sensitive data such as authorization headers, tokens and JWT-shaped strings.