Move `get_attrs` in try/except for validation handling

This enables raising ValidationError in get_attrs and catching as a 422.

Signed-off-by: Byron Ruth <b@devel.io>
Validation and initialize related data in service resources

Before the classes had to guess how to handle the argument. Now
the service resource correctly handles this.

Also, the dependence and direction attributes have been added to the
Edge service resources.

Signed-off-by: Byron Ruth <b@devel.io>
Add service resources for Relationship model

This also includes methods on Resource for accessing relationships.

Signed-off-by: Byron Ruth <b@devel.io>