Allow string entity reference resolvers to control the type setup

When writing a string entity reference resolver, you are currently bound to a fixed type setup/flow, depending on what enity type you are resolving.

Example: After Attachment comes Document; After Document comes Space, etc.

It would be useful to allow a resolver to specify its own type setup, so that it can still benefit from the current string reference parsing code (which is nicely done and properly handles escaped values, etc.). Of course, it would be the resolver's job to make sure that it produces valid references, i.e. if it skips parts of the standard type setup to provide defaults that make sense for anyone using it.

One usecase of this is the ability to resolve attachment references specified in "space attachment" form (i.e. without the .WebHome part), e.g. Space@file.ext which would be resolved to wiki:Space.WebHome@file.ext