interface for inclusion of 3rd party content in project's lookup. Typically, if the
project type allows composition of lookup from multiple sources, it will make a layer
location public where 3rd parties will register implementations of this interface.
implementations will be asked to create their additional project lookup based on the baseContext
passed as parameter. The content of baseLookup is undefined on this level, is a contract
of the actual project type. Can be complete lookup of the project type, a portion of it or
something completely different that won't appear in the final project lookup.
Each implementation is only asked once for it's lookup for a given project instance at the time
when project's lookup is being created.
baseContext - implementation shall decide what to return for a given project instance based on context
a Lookup instance that is to be added to the project's lookup, never null.