support default variables

Imported from tarantool/cartridge@6eb4a43

```
See: https://graphql.org/learn/queries/#default-variables
Note: there is a difference between explicitly passed "null" value
and just omitted variable. See graphql/graphql-spec#418

Closes tarantool/cartridge#866
```

graphql/execute.lua

@@ -5,6 +5,10 @@ local introspection = require(path .. '.introspection')
 local query_util = require(path .. '.query_util')
 local validate_variables = require(path .. '.validate_variables')
 
+local function error(...)
+  return _G.error(..., 0)
+end
+
 local function getFieldResponseKey(field)
   return field.alias and field.alias.name.value or field.name.value
 end
@@ -236,16 +240,26 @@ local function getFieldEntry(objectType, object, fields, context)
     argumentMap[argument.name.value] = argument
   end
 
+  local defaultValues = {}
+  if context.operation.variableDefinitions ~= nil then
+    for _, value in ipairs(context.operation.variableDefinitions) do
+      if value.defaultValue ~= nil then
+        local variableType = query_util.typeFromAST(value.type, context.schema)
+        defaultValues[value.variable.name.value] = util.coerceValue(value.defaultValue, variableType)
+      end
+    end
+  end
+
   local arguments = util.map(fieldType.arguments or {}, function(argument, name)
     local supplied = argumentMap[name] and argumentMap[name].value
 
     supplied = util.coerceValue(supplied, argument, context.variables,
       {strict_non_null = true})
-    if supplied ~= nil then
+    if type(supplied) ~= 'nil' then
       return supplied
     end
 
-    return argument.defaultValue
+    return defaultValues[name]
   end)
 
   --[[

graphql/rules.lua

@@ -410,7 +410,8 @@ function rules.variableDefaultValuesHaveCorrectType(node, context)
       if definition.type.kind == 'nonNullType' and definition.defaultValue then
         error('Non-null variables can not have default values')
       elseif definition.defaultValue then
-        util.coerceValue(definition.defaultValue, context.schema:getType(definition.type.name.value))
+        local variableType = query_util.typeFromAST(definition.type, context.schema)
+        util.coerceValue(definition.defaultValue, variableType)
       end
     end
   end