USD: Prototype point cloud reader.

Added 'Point Cloud' USD import option.

Added new USDPointsReader class with stub functions.
The reader creates a point cloud object and allocates
the points, but it does not set point poistions or
attributes.  The reader will also allocate a cache modifier
for time varying points.

Updated the stage reader to instantiate the USDPointsReader
during stage traversal whe converting the UsdGeomPoints
schema.

@@ -0,0 +104,4 @@
+  bke::SpanAttributeWriter<float3> positions_writer =
+      point_cloud->attributes_for_write().lookup_or_add_for_write_span<float3>(
+          "position", bke::AttrDomain::Point);
+  MutableSpan<float3> point_positions = positions_writer.span;

@@ -0,0 +106,4 @@
+          "position", bke::AttrDomain::Point);
+  MutableSpan<float3> point_positions = positions_writer.span;
+
+  for (size_t i = 0; i < positions.size(); i++) {

@@ -0,0 +118,4 @@
+   * custom spans for the attributes. */
+
+  std::vector<pxr::UsdGeomPrimvar> primvars = primvarsLoaded.GetPrimvarsWithValues();
+  for (auto pv : primvars) {

@@ -0,0 +143,4 @@
+      for (int i = 0; i < colors.size(); ++i) {
+        const pxr::GfVec3f &usd_color = colors[i];
+        primvar_writer.span[i] = ColorGeometry4f(usd_color[0], usd_color[1], usd_color[2], 1.0f);
+      };

@@ -0,0 +161,4 @@
+        continue;
+      }
+
+      for (int i = 0; i < values.size(); ++i) {