Feature Request: Add terminationGracePeriodSeconds field in liveness probe

/area API
/kind good-first-issue
/kind spec

## Describe the feature
The kubernetes pod spec allows us to mention `terminationGracePeriodSeconds` under livenessProbe for a specific container. This helps restarting a failed container. Current the knative service spec ignores this field.

docs for reference: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#probe-level-terminationgraceperiodseconds

My use case:
The liveness probe for the container fails and the container receives a sigterm. The container code went into a deadlock and doesnt respect gracefull shutdown. Now the container waits for pod `terminationGracePeriodSeconds` which is set by [knative](https://knative.dev/docs/serving/configuration/config-defaults/#revision-timeout-seconds). This is taking around 300 second for the pod to restart. I dont want this to setting because of request timeout, but want the container to restart as soon as liveness probe fails.