Accept stdin as a "data values file"

[pivotaljohn]

Describe the problem/challenge you have
I want to supply a plain YAML data values file on "standard input" (aka stdin) to my ytt invocation

(ex: https://kubernetes.slack.com/archives/CH8KCCKA5/p1631819726297200)

Describe the solution you'd like

$ generate-data-values | ytt -f config/ --data-values-file=-

where:

• generate-data-values is some process that yields (to stdout) a plain YAML that is suitable as a data values file.
• 🛑 If stdin is supplied both for a --file entry and --data-values-file, this should error out explaining that standard input can only be used once.

Anything else you would like to add:

• 🏅 This (subtly) also improves the use of ytt in Windows environment where the same line can be used in Powershell.
• 🤔 Consider the suggestion to explore how to reduce the total code paths while implementing this feature: can this kind of file handling (regardless of which flag is accepting the file) be centralized/turned into a core feature of the system?

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[pivotaljohn]

Maintainers have committed to staff this work. Changed priority accordingly.

[aaronshurley]

This will be available in the next version (v0.41.0).