Return a Single (RxJava) of type CheckResponse for those who want a Single instead of a callback

[s0h4m]

Return a Single<CheckResponse> when looking up for a feature like so

Toggle.with(context).check("custom_network_component").getLatest().defaultState(Toggle.ENABLED).start(checkResponse -> updateUiAfterResponse(checkResponse.featureName, checkResponse.state, checkResponse.featureMetaData, checkResponse.ruleMetadata, checkResponse.cached), exception -> {});