Unify argument validation

What problem does this feature proposal attempt to solve?

The native spec-compliant validation performed by webonyx/graphql-php (!, scalars) and Lighthouse/Laravel (@rules, @validator) run in different phases and return different error shapes.

This has lead to number of issues:

• https://github.com/nuwave/lighthouse/issues/345
• https://github.com/nuwave/lighthouse/issues/262

Which possible solutions should be considered?

type Query {
  foo(
    nonNull: ID!
    customScalar: CustomScalar
    required: String @rules(apply: ["required"]
    email: String @rules(apply: ["email"]
  ): ID
}

query EverythingWrong {
  # Missing nonNull and required
  foo(customScalar: "wrong-format", email: "not-an-email")
}

An optimal solution would combine all kinds of validation in a single, unified step and return a consistent format for which inputs failed validation.

{
  "errors": [
    {
      "message": "Validation failed for the field [foo]",
      "extensions": {
        "validation": {
          "nonNull": ["The nonNull field is required."],
          "customScalar": ["The customScalar field must be a valid CustomScalar."],
          "required": ["The required field is required."],
          "email": ["The email field must be a valid email."]
        }
      }
    }
  ]
}

Start by reviewing how webonyx/graphql-php handles native validation and how Lighthouse/Laravel process @rules and @validator, then compare the related issues #345 and #262. Use the sample schema and query to trace the separate phases. Done should be defined as a consistent validation step and error shape covering all listed input failures.