feat(posthog-server): Add `onlyEvaluateLocally` option

[dustinbyrne]

Description

For SDK parity, PostHogFeatureFlagOptions should have a onlyEvaluateLocally boolean flag which does not allow flag evaluation to make an API request.

[dmarticus]

@dustinbyrne does this make sense to keep on the feature flags board or is this more of a client libraries thing?

[dustinbyrne]

I'll own it. Taking it off the flags board.