rxjs/webSocket should support deserializer return Promise

Feature Request

rxjs/webSock should support deserializer return Promise

Is your feature request related to a problem? Please describe.

import { webSocket } from 'rxjs/webSocket'

const api = (options) => {
  const subject = webSocket(options);
  return {
    subscribe: (params: any) => {
      const observableA = subject.multiplex(
        () => params,
        () => ({ close: true }),
        (message: any) =>  {
          return message.requestId === params.requestId
        }
      );

      return observableA
    }
  }
}

const ws = api({
  url: 'ws://127.0.0.1:50055/ws/v2',
  protocol: 'json',
  binaryType: 'blob',
  deserializer: ({ data }) => {
    return new Promise((res, rej) => {
      const reader = new FileReader()
      reader.readAsText(data)
      reader.onload = () => {
        res(JSON.parse(reader.result))
      }
    })
  }
})

ws.subscribe({requestId: '1', data:{}})
  .subscribe((res) => {
    console.log('res', res);
  })

the deserializer have to return Promise because i have to resolve Blob, but socket.onmessage doesn't support deserializer function to return Promise