This change introduces a new stdin field to the Application spec under source.plugin.stdin. The stdin value is interpolated with standard build environment variables and then passed via standard input to the CMP's generate, discover, and parameters commands.

This allows passing large payloads to plugins without hitting exceeding limits on things passed as argument to execve, including environment variables:

• Linux MAX_ARG_STRLEN is 128 KB (a single argument, e.g. a single environment variable)
• Linux MAX_ARG is generally 2 MB

Both of these appear as fork/exec $path: argument list too long errors.

Fixes #17914

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