valid: Add NullableString

Add a NullableString method to allow users to differentiate between the
absence of a field and an explicitly provided null value.

valid: Add Errorf function

Add an Errorf function to create GraphQL errors without a validation
context.

valid: add OptionalBool

valid: fix nil values in OptionalString

valid: add generic optional interface

auth: force webhooks to read-only

valid: accept format strings

valid: Fix Sprintf call

valid: new module for input validation