Curves: Port endpoint selection node to new data-block

The node should be faster than in 3.1, for a few reasons:
- It doesn't need to calculate and allocate the curve offsets.
- It doesn't need to de-reference a pointer for each curve.
- The inputs are accessed from the virual arrays fewer times.

On top of that, I added two other performance improvements:
- The node is multi-threaded when there are many curves.
- There are generated special cases for single value and span inputs.

**Performance**
With a set position node affecting 1 million splines with a selection
based on this node, on an Intel i5 8250U (times are approximate):
| Before | After | Speedup |
| 760 ms | 60 ms | 13x     |

Differential Revision: https://developer.blender.org/D14233

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