support adding planning entities dynamically during a move

As the issue summary describes, some optimization problems can benefit from the ability to add planning entities dynamically during a move to make the search more efficient. This is the essence of the request and we shall elaborate on that in the comments.

One simple VRP use case we have for that is inserting waste collection facilities into waste collection truck routes, where the number of waste collection facilities to insert in each route is not known ahead of time - it may be just one or two per route, but may also be more. Without the ability to add these facilities dynamically during optimization, they have to be pre-generated in a fixed number before solving, which is not very convenient, especially if there are a lot of trucks and multiple waste collection facilities to choose from.