feat: upgrade logger

[tckeong]

This PR upgrade the original logger, add a new logger backend (logger driver). The new logger backend, CloudWatch Logger, enabling buffered delivery of your application logs to AWS CloudWatch using ExAws.

---

How It Works

1. Initialization
   On startup, this backend reads configuration options (:log_group, :log_stream, :level, :format, :metadata) to set up its internal state.

2. Buffering Logs
   When the application emits a log, the handle_event/2 callback formats it, attaches metadata, timestamps it, and buffers it.

3. Flush Trigger
   Logs are flushed in two cases:

   • The buffer reaches the configured @max_buffer_size (immediately triggers async flush).
   • A periodic timer (@flush_interval) sends a :flush_buffer message, flushing any pending logs.

4. AWS Auth Check
   Before sending, the logger verifies that AWS credentials and region are configured via environment variables. If missing, the flush is skipped (avoiding endless retry loops).

5. Retry Logic
   Logs are sent using ExAws.request/1, in %ExAws.Operation.JSON{} format. On failure, it retries up to @max_retries, with a delay of @retry_delay between attempts. An error is logged on each failure.

---

Configuration Options

Add to your config/*.exs:

config :logger,
  backends: [:console, {Cadet.Logger.CloudWatchLogger, :cloudwatch_logger}]

config :logger, :cloudwatch_logger,
  level: :info,
  format: "$time $metadata[$level] $message\n",
  metadata: [:request_id],
  log_group: "cadet-logs",
  log_stream: "#{node()}-#{:os.system_time(:second)}",

• :log_group / :log_stream - CloudWatch Logs group and stream names.
• :level, :format, :metadata – Same options as Logger.Formatter.

This PR integrates with Elixir’s Logger (:gen_event), offering buffered, retry-enabled delivery to AWS CloudWatch.