Custom resource controller actions and more

Added

• #12 Can now register routes for custom actions on a resource, using the actions() helper method when registering resources. See the PR for examples.
• The JsonApiController now has the Laravel AuthorizesRequests, DispatchesJobs and ValidatesRequests traits applied.
• #6 Resource class can now use conditional fields in their relationships. This works in the same way as conditional attributes: the resource's when() and mergeWhen() method should be used to add conditional relationships.
• #13 Added French translations for JSON:API errors generated by specification parsing and resource/query parameter validation.
• #7 Eloquent schemas now support default eager loading via their $with property.
• #15 When parsing a JSON:API document for compliance with the specification, the client will now receive a clearer error message if they submit a to-one relationship object for a to-many relationship (and vice-versa).

Changed

• #2 BREAKING Improved the extraction of existing resource field values when constructing validation data for update requests:
  • The existingAttributes() and existingRelationships() methods on the resource request class has been removed. If you need to modify the existing values before the client values are merged, implement the withExisting() method instead. This receives the model and its JSON representation (as an array).
  • The mustValidate() method must now be called on a schema relationship field. (Previously this was on the resource relation.) By default, belongs-to and morph-to relations will be included when extracting existing values; all other relations will not. Use the mustValidate() or notValidated() method on the schema relation to alter whether a relation is included in the extracted values.