Need BLTouch and Z-Stop to improve bed leveling

The stepper_z.endstop_pin can be configured for the BLTouch or the z-stop switch, but not both. It would be prudent to support both. The reason is that manually leveling the bed periodically ensures more accurate prints, but it also results in the slow lifting of the bed over time. The goal should be to keep the bed as low as possible, as that keeps the bed-leveler tension springs under tension. When the springs are under tension, the bed leveling screws are less likely turn during printing.

It should be possible to zero out the bed by lowering it all the way, then lower the extruder until the z-stop switch is triggered. Then, manually leveling the bed from here. In this scenario, the bed leveling springs are under the most tension, providing the best overall stability through the course of the print. However, it should also be considered safe to use the BLTouch as a z-stop for quick homing of the printer between prints w/o requiring a full level of the bed again. In this scenario, the z-stop will double as gantry crash protection, in the event that the bed is too low for the BLTouch probe to detect the bed (as is the case on my printer) and not force me to have to go through multiple printer.cfg changes, just to re-zero the bed leveling.