Allow SpeculationRules prefetch to ServiceWorker-controlled URLs

SpeculationRules prefetch can be issued to URLs that are controlled by
ServiceWorker. However, legacy code did not allow it and canceled the prefetch
requests. This policy enables to control the behavior.

Setting this policy to Enabled or not set allows SpeculationRules prefetch to
ServiceWorker-controlled URLs (if the PrefetchServiceWorker feature flag is
enabled). This is the current default behavior and is aligned with the
specifications.

Setting this policy to Disabled disallows SpeculationRules prefetch to
ServiceWorker-controlled URLs. This is the legacy behavior.

This policy is intended to be temporary and will be removed in the future.