feat(graphql): support one of input objects

[toy0605]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #3687

What is the new behavior?

Code First

Code

// New `isOneOf` property
@InputType({ isOneOf: true })
export class UserByInput {
  @Field(type => ID)
  id: string;

  @Field(type => String)
  email: string;

  @Field(type => String)
  username: string;
}

@Resolver(of => User)
export class UserResolver {
  @Query(type => User)
  user(@Args("by") by: UserByInput) {
    // ...
  }
}

Result (Auto Generated GraphQL Schema)

input UserByInput @oneOf {
  id: ID
  email: String
  username: String
}

type Query {
  user(by: UserByInput!): User
}

Schema First

GraphQL Schema

input UserByInput @oneOf {
  id: ID
  email: String
  username: String
}

type Query {
  user(by: UserByInput!): User
}

Code (Auto Generated Type)

export class UserByInput {
  id: string;
  email: string;
  username: string;
}

// ...

export abstract class IQuery {
    abstract user(by: UserByInput): Nullable<User> | Promise<Nullable<User>>;
}

type Nullable<T> = T | null;

Resolver

@Resolver("User")
export class UserResolver {
  @Query()
  user(@Args("by") by: UserByInput) {
    // ...
  }
}

Does this PR introduce a breaking change?

• Yes
• No

Other information