Move saved objects helper to new platform

[flash1293]

There are some helpers around saved objects currently located in src/legacy/ui/public/saved_objects/saved_object_loader.ts and src/legacy/ui/public/saved_objects/saved_object.ts.

Those together with the helper utilities in src/legacy/ui/public/saved_objects/helpers should get moved into the newly created saved_objects NP plugin. The createSavedObjectClass helper and SavedObjectLoader class should be exposed as static exports from the top level of the plugin. All consumers should switch to the new location directly (no proxying via re-export file still in ui/public).

The tests located in src/legacy/ui/public/saved_objects/__tests__ should get converted also moved over in the same step. The test for SimpleSavedObject can be moved into core (the implementation is already located there).

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)