[PP-1] ImageToolkitOperations need traits

Problem/Motivation

The GD image toolkit operations implement the ::arguments() and ::validateArguments() methods.
This code is mostly boilerplate, and other toolkits need to re-enter the same code because their operations are extended from a base class that, basically, does nothing more than redefine the type of the ::getToolkit() method to the actual toolkit.
For this reason, toolkit operations can not inherit from an independent 'operation base class', so code in the two methods indicated above needs to be replicated.

Proposed resolution

Introduce a trait for each operation (e.g. CropTrait, RotateTrait, ScaleTrait, etc.) with ::arguments() and ::validateArguments() implementations, so that each toolkit operation can use its own trait and only implement a toolkit specific ::execute() method. This approach is already implemented in the contrib Image Effects → module.

Remaining tasks

Wait for 📌 Move GD-specific code from Rotate::validateArguments() to Rotate::execute() Needs work .

User interface changes

None

API changes

To be determined

Data model changes

None