Support an input_delay specification when scheduling a pipeline

[hongted]

Timing constraints for input port to register paths and register to register paths may be different. Additional scheduler options should be added and the scheduler enhanced so that these paths can be constrained differently.

At the most basic, an additional input_delay_in_ps can be added to the options. That delay should be added to all paths which are expected to become input port to register paths.

[hongted]

Note that this is not exactly the same as adding an input delay on each receive node. Multiple input channels may be sequentially dependent in the IR, but may be partly parallelized when realized.

[hongted]

Duplicate of https://github.com/google/xls/issues/575