Change return type for oracle getOnlineTimeout to return Big

`OracleApi.getOnlineTimeout()` currently returns a `number` which can lead to errors for particularly big values - which can be set in test environments to avoid oracle values from timing out.

Change the return type to `Big` to avoid that issue.

Consider adding a new endpoint and deprecating the current one to create a breaking API change (together with other ones) at a later stage.