title: prisma Order by relation has only _count property. Can not order by relation fields
body: <p>consider following Prisma schema:</p>
  <pre><code>model Conversation {
    id           Int                         @id @default(autoincrement())
    createdAt    DateTime                    @db.Timestamp(6)
    messages     ConversationMessage[]
  }
  
  model ConversationMessage {
    id             Int                     @id @default(autoincrement())
    text           String                  @db.VarChar(1000)
    sentAt         DateTime                @map(&quot;sent_at&quot;) @db.Timestamp(6)
    conversationId Int?                    @map(&quot;conversation_id&quot;)
    userId         Int?                    @map(&quot;user_id&quot;)
    conversation   Conversation?           @relation(fields: [conversationId], references: [id])
    sender         User?                   @relation(fields: [userId], references: [id])
  }
  </code></pre>
  <p>I want to run such query so that I get a list of conversations ordered by date of their messages, i.e. the ones with new messages first.</p>
  <pre><code>prisma.conversation.findMany({
      orderBy: {
          messages: {
              sentAt: 'desc'
          }
      },
      ...
  })
  </code></pre>
  <p>But the only way that I can query now is like this, i.e. relation has only <code>_count</code> property somehow.</p>
  <pre><code>prisma.conversation.findMany({
      orderBy: {
          messages: {
             '_count': 'desc'
          }
       },
       ...
  })
  </code></pre>
  <p>Environment &amp; setup</p>
  <pre><code>
      OS: Mac OS,
      Database: PostgreSQL
      Node.js version: v12.19.0
  
  </code></pre>
  <p>Prisma Version</p>
  <pre><code>prisma               : 2.24.1
  @prisma/client       : 2.24.1
  Current platform     : darwin
  Query Engine         : query-engine 18095475d5ee64536e2f93995e48ad800737a9e4 (at node_modules/@prisma/engines/query-engine-darwin)
  Migration Engine     : migration-engine-cli 18095475d5ee64536e2f93995e48ad800737a9e4 (at node_modules/@prisma/engines/migration-engine-darwin)
  Introspection Engine : introspection-core 18095475d5ee64536e2f93995e48ad800737a9e4 (at node_modules/@prisma/engines/introspection-engine-darwin)
  Format Binary        : prisma-fmt 18095475d5ee64536e2f93995e48ad800737a9e4 (at node_modules/@prisma/engines/prisma-fmt-darwin)
  Default Engines Hash : 18095475d5ee64536e2f93995e48ad800737a9e4
  Studio               : 0.397.0
  Preview Features     : orderByRelation
  
  </code></pre>
  <p>Thank You!</p>