Fleet: disable package cache for development purposes

While developing integrations it's a bit irritating to restart Kibana instance as some parts of a package are being cached.

For example - when modifying manifest's description, we don't have to restart Kibana. It's enough to reboot Package Registry with updated package.

But - when modifying policy templates, we have to artificially bump up the package version to make sure Kibana won't cache it (or rather load it for the first time).

The goal of this issue is to introduce a configuration property (option?) to disable package caching at all (only for development purposes).

cc @jen-huang @sayden