validation: add input object definition to compiler's context

[lrlna]

• add input object definition to source database
• add validation
  • [x] unique name
  • [ ] circular reference validation, spec
  • [x] input field names must be unique
  • [ ] If an Input Object references itself either directly or through referenced Input Objects, at least one of the fields in the chain of references must be either a nullable or a List type. (needs clarification)