Option to choose a different body field to get the node data from

If anyone needs this, please provide a MR with tests. Thank you!

Could we add the information that the body field is required for the module to work to the project page and readme?

@anruether definitely yes! I'm also absolutely not against making this editable - the point is just about complexity, because we either have to set the option per bundle or ensure the field exists in the enabled bundles.

I also don't like the hard-coded body field that much, but it's at least a simple >80% solution. What about maybe introducing the setting in config first and do the UI later? Anyone interested to implement that?