implement missing GraphQL spec validations

[imor]

The following validations are not currently performed by pg_graphql:

https://spec.graphql.org/October2021/#sec-Field-Selections
https://spec.graphql.org/October2021/#sec-Leaf-Field-Selections
https://spec.graphql.org/October2021/#sec-Argument-Names
https://spec.graphql.org/October2021/#sec-Argument-Uniqueness
https://spec.graphql.org/October2021/#sec-Fragment-Name-Uniqueness
https://spec.graphql.org/October2021/#sec-Fragments-On-Composite-Types
https://spec.graphql.org/October2021/#sec-Fragments-Must-Be-Used
https://spec.graphql.org/October2021/#sec-Fragment-spread-is-possible
https://spec.graphql.org/October2021/#sec-Input-Object-Field-Uniqueness
https://spec.graphql.org/October2021/#sec-Directives-Are-Defined
https://spec.graphql.org/October2021/#sec-Directives-Are-In-Valid-Locations
https://spec.graphql.org/October2021/#sec-Directives-Are-Unique-Per-Location
https://spec.graphql.org/October2021/#sec-Variable-Uniqueness
https://spec.graphql.org/October2021/#sec-Variables-Are-Input-Types
https://spec.graphql.org/October2021/#sec-All-Variables-Used
https://spec.graphql.org/October2021/#sec-All-Variable-Usages-are-Allowed

None of them are any blockers because the user has always an option to correct their query to avoid the behaviour due to missing validation but we should consider implementing them for better error messages and spec compliance.