Add the ability to pass data to processes over stdin using ProcessLauncher

[daveaglick]

There are scenarios where cli applications expect content to be provided. ProcessLauncher should support this by allowing a Stream that will get fed to the processes over stdin.

Additionally, this feature should be exposed through the StartProcess module to take configuration delegates that provide such content (for example, to feed the content of each document to a process-per-document).

See the discussion in https://github.com/orgs/statiqdev/discussions/181.