Added IDisabledTrait & UpgradableTrait with upgrade levels support

 * IDisabledTrait for identifying (and filtering) disabled traits
 * UpgradeEnabledDisabledTrait provides an abstract base for enabling by upgrade and disabling by upgrade
We will eventually also want to support an `UpgradeTypeInit` so that actors can be created with upgrades enabled, but that should be deferred to a later pr so as not to further delay this.