Extend BackendTrafficPolicy Rate Limiting to Support Custom Time Intervals

*Description*:
Currently, BackendTrafficPolicy rate limiting only supports predefined time units (Second, Minute, Hour, Day). This limitation forces users to work around scenarios where custom intervals (e.g., 30 seconds, 5 minutes) are required for precise rate limiting control.

*Current Limitation*
```yaml
# ❌ Currently NOT supported
rateLimit:
type: Global
global:
rules:
- limit:
requests: 1500
unit: Second
interval: 30 # Custom interval not supported
```

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