Support unknown variable evaluation

Expose a callback to handle unknown variables found in hcl syntax. This is
useful in situations where the variables aren't known statically upfront.

eval_context.go

@@ -11,6 +11,10 @@ type EvalContext struct {
 	Variables map[string]cty.Value
 	Functions map[string]function.Function
 	parent    *EvalContext
+
+	// UnknownVariable handles when an unknown variable is referenced.
+	// This handler is experimental and may change in the future.
+	UnknownVariable func(Traversal) (cty.Value, error)
 }
 
 // NewChild returns a new EvalContext that is a child of the receiver.

traversal.go

@@ -70,7 +70,12 @@ func (t Traversal) TraverseAbs(ctx *EvalContext) (cty.Value, Diagnostics) {
 
 	thisCtx := ctx
 	hasNonNil := false
+	var unknownHandler func(Traversal) (cty.Value, error)
 	for thisCtx != nil {
+		if unknownHandler == nil && thisCtx.UnknownVariable != nil {
+			unknownHandler = thisCtx.UnknownVariable
+		}
+
 		if thisCtx.Variables == nil {
 			thisCtx = thisCtx.parent
 			continue
@@ -83,6 +88,13 @@ func (t Traversal) TraverseAbs(ctx *EvalContext) (cty.Value, Diagnostics) {
 		thisCtx = thisCtx.parent
 	}
 
+	if unknownHandler != nil {
+		v, err := unknownHandler(t)
+		if err == nil {
+			return v, nil
+		}
+	}
+
 	if !hasNonNil {
 		return cty.DynamicVal, Diagnostics{
 			{