codegen: support predicate arguments

[hgiasac]

Add the ndc struct tag to support predicate arguments. If there is a schema.Expression field with ndc:"predicate=<object-name>" tag in the argument struct it will be generated as a Predicate type.

type CreateAuthorArguments struct {
	Where schema.Expression `json:"where" ndc:"predicate=Author"`
}

{
  "arguments": {
    "where": {
      "type": {
        "type": "nullable",
        "underlying_type": {
          "object_type_name": "Author",
          "type": "predicate"
        }
      }
    }
  },
  "name": "createAuthor",
  "result_type": {
    "type": "nullable",
    "underlying_type": {
      "name": "CreateAuthorResult",
      "type": "named"
    }
  }
}

[github-actions]

Package	Line Rate	Health
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/command/internal	83%	✔
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/command	34%	❌
github.com/hasura/ndc-sdk-go/cmd/hasura-ndc-go/version	43%	➖
github.com/hasura/ndc-sdk-go/connector	54%	➖
github.com/hasura/ndc-sdk-go/example/reference	57%	➖
github.com/hasura/ndc-sdk-go/ndctest	51%	➖
github.com/hasura/ndc-sdk-go/scalar	97%	✔
github.com/hasura/ndc-sdk-go/schema	48%	➖
github.com/hasura/ndc-sdk-go/utils	35%	❌
Summary	52% (6738 / 12842)	➖

Minimum allowed line rate is 40%