Provider functions in variable validations (#1689)

Signed-off-by: Christian Mesh <christianmesh1@gmail.com>

internal/command/e2etest/testdata/functions/main.tf

@@ -7,6 +7,15 @@ terraform {
 	}
 }
 
+variable "number" {
+	type  = number
+	default = 1
+	validation {
+		condition = provider::example::echo(var.number) > 0
+		error_message = "number must be > ${provider::example::echo(0)}"
+	}
+}
+
 output "dummy" {
 	value = provider::example::echo("Hello Functions")
 }