execution on generated layers after the algorithm (or parent
model) completes
This commit adds an interface for layer post-processing handlers
for execution following a processing algorithm operation.
Post-processing of a layer will ONLY occur if that layer is set
to be loaded into a QGIS project on algorithm completion.
Algorithms that wish to set post-processing steps for generated
layers should implement this interface in a separate class
(NOT the algorithm class itself!) and implement a method
to handle the layer post-processing.
This method always runs in the main thread and can be used to
setup renderers, editor widgets, metadata, etc for the given layer.
Fixes#17961
