cancellation file support for bluebox

[wjblanke]

This change allows a file path to be passed in which must exist for a GMP slow blue box VDF to continue running. This enables the calling process to abort the VDF by removing the file (a temporary file so crashes or bad cleanups do not affect future runs). This is important to allow bluebox timelord processes to exit cleanly and not require termination.

Associated chia blockchain pr (without VDF update so broken) https://github.com/Chia-Network/chia-blockchain/pull/18642