jsonconfig: Additional details about input variables

The JSON object describing an input variable can now include two additional
properties:

- "type" provides a JSON representation of the variable's type constraint,
  if one is set. Omitted if either there is no constraint declared at all
  or if it's set to "any", which are equivalent and both mean that the
  type is completely unconstrained.

  This uses the standard cty representation of a type constraint, which
  matches how OpenTofu already describes types in the provider protocol,
  in state snapshots, and in saved plan files.
- "required" directly represents whether callers are required to provide
  a value for the variable. This is technically redundant since it is
  set to true unless "default" is also set, but this avoids the need for
  consuming software to reimplement this rule and potentially allows us to
  make this rule more complicated/subtle in future if needed.

For some reason the documentation about the JSON configuration
representation did not previously mention the "variables" property at all,
so this adds documentation for both the new properties and the pre-existing
properties.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

website/docs/internals/json-format.mdx

@@ -422,6 +422,54 @@ Because the configuration models are produced at a stage prior to expression eva
   // as the root of a tree of similar objects describing descendent modules.
   "root_module": {
 
+    // "variables" describes the input variable configurations in the module.
+    "variables": {
+
+      // Property names here are the input variable names
+      "example": {
+        // "type" describes the type constraint of the input variable, if any.
+        // This property is omitted for an unconstrained input variable.
+        //
+        // When present, its value is either a single string representing a
+        // primitive type, or an array with two or three elements describing a
+        // complex type:
+        // - "string", "number", or "bool" for the primitive types.
+        // - ["list", <type>] for a list type, where the second array element
+        //   is the list element type described in the same notation. The
+        //   collection type kinds are "list", "map", and "set".
+        // - ["object", <attributes>] for an object type, where the second
+        //   array element is a JSON object describing the object attributes
+        //   and their associated types. For an object type with optional
+        //   attributes, the array has a third element that is a JSON array
+        //   listing the attributes that are optional.
+        // - ["tuple", <elements>] for a tuple type, where the second array
+        //   element is a JSON array describing the tuple element types.
+        "type": "string",
+
+        // "default" is the default value of the input variable, serialized
+        // as JSON using the same mappings as OpenTofu's built-in "jsonencode"
+        // function.
+        "default": "Example",
+
+        // "required" is included and set to true if callers are required to
+        // provide a value for this variable, or omitted if it is optional.
+        "required": true,
+
+        // "description" is the author-provided description associated with
+        // this input variable, if any.
+        "description": "Example",
+
+        // "sensitive" is included and set to true if the input variable is
+        // declared as being "sensitive", or omitted if not.
+        "sensitive": true,
+
+        // "deprecated" is included and set to a deprecation message for
+        // any input variable that is declared as deprecated, or omitted for
+        // non-deprecated input variables.
+        "deprecated": "Example",
+      }
+    },
+
     // "outputs" describes the output value configurations in the module.
     "outputs": {