Add IOptionsMonitor registration to ConfigureValidatableSetting

[pecigonzalo]

When using Options, its quite handy to have some options be able to leverage reloading. Those require being added constructed as Scoped as otherwise they are never reloaded (as we call to CurrentValue).

[pecigonzalo]

Hey @andrewlock just looping back here, is this something that you could see being merged? Otherwise I would consider a hard fork.