support both realtime only subscriptions and subscriptions with LAD cache

[davetsay]

We currently get LAD upon subscribing to yamcs endpoints. This is useful to limit requests to yamcs. However, there are times when we only want to get realtime subscription data.

Describe the solution you'd like
Add a flag to either get realtime or realtime w/ LAD subscriptions.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[akhenry]

Let's talk about this. LAD requests have recently changed so that the latest value is retrieved by a different mechanism, so we could just remove the flag that returns the latest value via WebSocket. If nothing else, it should be returned via the "latest" data path anyway. So this flag may not be necessary.