Import API types from planx-core for key data structures (FlowGraph, Node, Breadcrumbs etc) #2405
Description
Currently, API types are defined that don't quite match their stricter planx-core counterparts (see comment here #2375 (comment)).
Doing this would allow us to work more safely, and standardise how we handle these data structures.
Where possible, I'm adding the JSDoc @deprecated to types I know need to be tidied up.