Feature: Point cloud expression builder for Processing framework

A new point cloud expression builder widget is available within the GUI to create filters for point clouds in a user friendly manner consistent with the existing QGIS Expression Builder interfaces.
Within the QGIS API, the QgsProcessingParameterExpression parameter has been extended with the types Qgis and PointCloud. If parameter has a PointCloud type, it will use the point cloud expression builder widget. To maintain backward compatibility, by default an expression parameter will use the Qgis expression type.
The QgsPointCloudExpression class was also extended with a new method to convert QGIS point cloud expressions to PDAL expressions.
Existing PDAL algorithms using expression filters have been updated to use expression parameters instead of strings.

This feature was funded by Point cloud processing and 3D data enhancements crowdfunding

This feature was developed by Alexander Bruy