Animation: Merge Push/Relax Rest Pose operators

This patch merges the two operators `Push Pose from Rest Pose`
and `Relax Pose from Rest Pose` into one
new operator `Blend Pose with Rest Pose`

The operator has a default range of -1/1 with a default of 0.
The functionality of `Relax Pose from Rest Pose` is on the positive axis
and `Push Pose from Rest Pose` on the negative.
This is a breaking change so ideal for 4.0

Pull Request: https://projects.blender.org/blender/blender/pulls/108309

scripts/startup/bl_ui/space_view3d.py

@@ -3822,8 +3822,7 @@ class VIEW3D_MT_pose_slide(Menu):
     def draw(self, _context):
         layout = self.layout
 
-        layout.operator("pose.push_rest")
-        layout.operator("pose.relax_rest")
+        layout.operator("pose.blend_with_rest")
         layout.operator("pose.push")
         layout.operator("pose.relax")
         layout.operator("pose.breakdown")