[ENHANCEMENT]: add @unique constraint in GraphQL Schema

[rderbier]

Use case and current behavior

GraphQL schema has @id but not unique constraint.

Enhancement

add @unique directive at field level. From : https://github.com/dgraph-io/dgraph/issues/8179

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Multiple Uniques or @unique

type User {
  id: ID! @id
  username: String! @unique
  email: String! @unique
}

That would be the equivalent of SQL

ADD CONSTRAINT UNIQUE (username)
ADD CONSTRAINT UNIQUE (email)

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