Support custom intervals for rate limit

Title: Support custom intervals for rate limit

Description:

Currently, 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

        rate_limit:
          requests_per_unit: 5
          unit: SECOND

Expected:

        rate_limit:
          requests_per_unit: 5
          unit: SECOND
          interval: 30

Envoy Gateway issue: https://github.com/envoyproxy/gateway/issues/6486