Allow passing a seed to /place jigsaw

The /place jigsaw command currently uses some combination of world seed and x/z coordinates to select and place pieces. This means that if the command is always called at the same location, it always generates the same structure.

While this is useful for the custom world generation system to ensure consistency between two saves with the same seed, it makes the utility of the command worse, as it cannot be used where the location is static e.g. for a minigame, where the arena has to stay in the same place.

I propose adding an optional parameter to the command, that allows a user to either provide a seed, or use rng not tied to the save/coordinates (potentially storing the resultant seed value somewhere for later use?)