Add new page id and token to bot service Facebook channel

**Is your feature request related to a problem? Please describe.**
I don't know how to add new Facebook page id and token to an existing bot channel registration.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
From azure cli, it should describe how to update Facebook channel with new page id and token, currently it only has `az bot facebook create` which creates a new channel.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.
Base on https://github.com/Microsoft/BotBuilder/issues/1495#issuecomment-408584611, we can have multiple pages connected to a bot channel registration but it's unclear how to do it using the cli