Add a path to feed bytes to the stdin of a Process

[svenier]

In our build flow we have some commands that read their input from stdin. With the current Process object, we need to setup and run a shell command instead of directly running our desired command to redirect input from a file.

If stdin were handled symmetrically to how stdout/stderr work, we could just directly invoke our desired command without the overhead (populating the sandbox with everything needed to run the shell) or risk (users stuffing shell metacharacters) of setting up a shell.

See https://pantsbuild.slack.com/archives/C01CQHVDMMW/p1763760163111359 for a long conversation about this.

[edit to fix typos]

[benjyw]

Makes sense. So next step would be a quick comment here on the design direction. E.g., is the stdin data an in-memory bytes that lives on the Process object? That's probably a good place to start, assuming the stdin data isn't huge.

[svenier]

Yes, that is exactly what I was thinking.