title: Typeorm column value from sanitized other column value
body: <p>I have this <code>User</code> class that uses typeorm. Is it possible to use the column transformer </p>
  
  <pre><code>@Entity()
  class User {
    @Index(ColumnIndex.UNIQUE_EMAIL, { unique: true })
    @IsEmail()
    @Column({ type: PostgresColumn.VARCHAR })
    email: ReadOnlyColumn&lt;string&gt;
  
    @Field()
    @IsEmail()
    @Column({ type: PostgresColumn.VARCHAR })
    authoredEmail: string
  }
  </code></pre>
  
  <p>Given a corresponding repository I'd like to be able to create like this:</p>
  
  <pre><code>user.create({
   authoredEmail: "Thomas.mEoW@gmail.com"
  })
  </code></pre>
  
  <p>Somehow i'd like to "normalizeEmail" the <code>email</code> column <code>Sanitizer.normalizeEmail(str, isLowercase);</code>. Is this possible to take place in the Entity using transformer?</p>