Crawler: Make the interface more generic

What

Change the type of the crawler functions so that it takes a more generic object with less methods/fields that are unused.

Why

To make it compatible with the context we have from the DAL node code.

How

Only rpc calls are needed, so we use an object that knows how to handle calls. By doing so, the chain field was missing that is provided at creation time.

For the current callers, it is not an issue since it is just a field that we explicit.