[Feature] Expose retry attempt count to RetryPolicy::backoff_hint

### Component

transports

### Describe the feature you would like

RetryPolicy::backoff_hint only receives the error:

` fn backoff_hint(&self, error: &TransportError) -> Option;`

When it returns None, RetryBackoffService falls back to a constant initial_backoff on every attempt — there is no exponential growth:

```
let backoff_hint = this.policy.backoff_hint(&err);
let next_backoff = backoff_hint.unwrap_or_else(|| this.initial_backoff());
```

Without access to the current attempt number, a custom policy cannot implement standard exponential backoff, e.g.:

` next_backoff = initial_backoff * (1 + backoff_coefficient)^(num_retries - 1)`

Proposal: pass the attempt count to the policy, e.g.

` fn backoff_hint(&self, error: &TransportError, num_retries: u32) -> Option;`

(or a context struct to keep it extensible). Alternatively, implement exponential backoff with a max ceiling directly in RetryBackoffService when no hint is
returned.

### Additional context

Downstream use case: hoprnet/blokli — custom retry policy with configurable initial_backoff, backoff_coefficient, max_backoff, currently unable to apply
them (hoprnet/hoprnet#7140).