Listen CLI API for team events

The `keybase chat api-listen` interface is a great way to listen for incoming chat events as they happen.

It would be nice if there were also a way to wait for incoming team events. For example, I'd love to listen for an event when someone requests access to a team, so that I can automatically add them if they meet certain criteria.

The GUI gets notifications when team access is requested, so it seems like the ability is just there, but not exposed in the CLI yet.

It would be nice to be able to get other team events, like people leaving the team, or having their access levels changed.

Additionally, it would be awesome if there were a single events API via which we could subscribe to multiple types of events. (team + chat + etc.)

Start by examining the existing `keybase chat api-listen` CLI interface and how the GUI receives team access notifications. Define the supported team event types and whether a combined chat-and-team subscription is in scope; done should be a documented CLI API that exposes the agreed events.

Written by the indexing model from the issue text.