Support warmup aggression values smaller than 1 in DestinationRule

[arnogeurts]

Describe the feature request

With #3215 the aggression and min_weight_percent settings are added to the warmup of the DestinationRule. However, in that feature the aggression is restricted to values in the range [1, +∞), whereas Envoy supports values in the range [0, +∞). This feature request adds support for the full range of the aggression settings in Envoy.

Describe alternatives you've considered

Affected product area (please put an X in all that apply)

[ ] Configuration Infrastructure
[ ] Docs
[ ] Installation
[x] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience

Additional context