DataConsumer: Add addSubchannel() and removeSubchannel()

[ibc]

Details

• Problem with dataConsumer.setSubchannels(subchannels) is that it's an async method and dataConsumer.subchannels getter doesn't get the updated value until setSubchannels() completes.
• So next code is a bit problematic:

  dataConsumer.setSubchannels([ 1, 2, 3 ]); // Note that we are not awaiting for it to complete.
  
  // Now let's add subchannel 4:
  dataConsumer.setSubchannels([ ...dataConsumer.subchannels, 4 ]);

• Resulting subchannels after this code are [ 4 ] rather than [ 1, 2, 3, 4 ] as one may expect.
• So we are adding addSubchannel() and removeSubchannel() methods to make this more easier:

  dataConsumer.setSubchannels([ 1, 2, 3 ]); // Note that we are not awaiting for it to complete.
  
  // Now let's add subchannel 4:
  dataConsumer.addSubchannel(4);

• Resulting subchannels after this code are [ 1, 2, 3, 4 ].