Follow-up to #2406749: Use a link field for custom menu link
Problem/Motivation
In the prior issue we are storing the user input by using a link field. The link field itself just has a URI, title and options stored.
However, for menu links referencing a node or other entity we should use a entity: URI. We don't need to do discovery and re-resolve those paths.
For links using a user-path: scheme we need to resolve the path every time we do menu rebuild - probably via plugin discovery as a way to re-process them
Proposed resolution
make these plugins participate in discovery and resolve the route then.
In order to easily find them, add a boolean base field that's not exposed in the UI, like "needs discovery"
Remaining tasks
Decide on implementation, do it.
User interface changes
N/A
API changes
Change to base table of entity, implement discovery for menu link content for the subset that need it.