feat: Sharded pub/sub support via dedicated subscribers

[dmaier-redislabs]

The previous implementation used one subscriber connection per cluster instance, but the command SSUBSCRIBE <channel> needs to be executed against a node that is responsible for the hash slot that is derived from the channel name.

This PR adds a ClusterSubscriberGroup whereby each subscriber in the group is responsible for one shard.

[bobymicroby]

There are some tests that need attention as well:

2) cluster:spub/ssub
       should receive messages:
     Uncaught TypeError: Cannot read properties of undefined (reading 'writable')
      at MockServer.write (test/helpers/mock_server.ts:139:11)
      at /home/runner/work/ioredis/ioredis/test/functional/cluster/spub_ssub.ts:26:13
      at tryCatcher (node_modules/standard-as-callback/built/utils.js:12:23)
      at /home/runner/work/ioredis/ioredis/node_modules/standard-as-callback/built/index.js:21:53
      at processTicksAndRejections (node:internal/process/task_queues:95:5)
  3) cluster:spub/ssub
       supports password:
     Uncaught AssertionError: expected undefined to deeply equal 'abc'
      at MockServer.handler (test/functional/cluster/spub_ssub.ts:66:31)
      at JavascriptRedisParser.returnReply (test/helpers/mock_server.ts:96:54)
      at JavascriptRedisParser.execute (node_modules/redis-parser/lib/parser.js:544:14)
      at Socket.<anonymous> (test/helpers/mock_server.ts:113:16)
      at Socket.emit (node:events:517:28)
      at Socket.emit (node:domain:489:12)
      at addChunk (node:internal/streams/readable:368:12)
      at readableAddChunk (node:internal/streams/readable:341:9)
      at Socket.Readable.push (node:internal/streams/readable:278:10)
      at TCP.onStreamRead (node:internal/stream_base_commons:190:23)
      
     ``` 

[github-actions]

🎉 This PR is included in version 5.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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