Add subcommands to `forest-cli` to handle channels

[elmattic]

Issue summary

This is a follow up of https://github.com/ChainSafe/forest/pull/3978.

Right now, we're using some javascript code to exercise our pubsub RPC code. We would like to have some subcommand achieving similar functionality.

Task summary

• [ ] Add a subcommand to open a channel
• [ ] Add a subcommand to cancel a subscription

Acceptance Criteria

Other information and links

[lemmih]

forest-cli should not have a subcommand for opening a channel. The Forest RPC library should have support for channels. Otherwise, we cannot support the chain_notify method.

[lemmih]

Closing as not-actionable.