configs: Include context when variable default has nested problem

[apparentlymart]

Previously this error message was including only the main error message and ignoring any context about what part of a potential nested data structure it arose from.

Now we'll use our usual path formatting utility to introduce additional context whenever a problem arises with a nested value.

This fixes #27773 by making its error message now include the problematic attribute path:

╷
│ Error: Invalid default value for variable
│ 
│   on variable-default-validation-context.tf line 6, in variable "foo":
│    6:   default = {
│    7:     "memap" = {
│    8:        abc = "i am a string"
│    9:        iamme = "one two three"
│   10:     }
│   11:   }
│ 
│ This default value is not compatible with the variable's type constraint:
│ ["memap"].abc: a bool is required.
╵

[github-actions]

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[github-actions]

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