Variable Validation: allow functions in error_message

[alexjurkiewicz]

Here's a validation test I wrote for a MySQL username:

validation {
    condition     = length(var.username) > 16
    error_message = "The username cannot be more than 16 characters (first 16 are ${trim(var.username, 0, 16)})."
  }

I wanted to give a hint to the operator about the longest string allowed, to make their life easier. But you can't use functions in error_message. Please add support for the above use-case.

[joe-a-t]

duplicate of #24160

[github-actions]

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