Non-Copy Datastream Migrations

[barmintor]

I'm working on a fork that would allow registration of Mover classes for content migrations by attached_file name, and wanted to open a conversation here. The idea is that, for a target class that (<>ldp:contains 'foo','bar'), the ObjectMover has a map (defaulting to DatastreamMover) of Mover impls that might perform some arbitrary action for the migration of that content. For example, it may go grab the contents for 'foo' from another object that won't be migrated, or just insert default content.

@mjgiarlo @Cam156 @awead

[barmintor]

my WIP fork is at https://github.com/barmintor/fedora-migrate/tree/content-converter