Upgrade ReadyValidPipeline to support additional stalls

[mkorbel1]

Motivation

The ReadyValidPipeline currently hijacks access to stall signals. This is annoying if you want to stall pipeline stages for other purposes than just backpressure on the end of the pipeline.

Desired solution

Add ability to provide additional stalling signals per-stage even for ReadyValidPipeline

Alternatives considered

No response

Additional details

This might fall under a larger refactor of Pipelines to use some class instead of just Functions

[mkorbel1]

Also, think about flushing