Idempotency Key Validation

The idempotency can be made configurable based on the users preferences.

- Configurable key validation (format, length, pattern) can be made available to set, something like:

```yaml
avoonce:
  idempotency:
    key-pattern: "^[a-fA-F0-9\\-]{36}$"  # UUID format
    max-key-length: 128
```
Also the key validation prevents abuse (overly long keys, injection attempts)
