Improve UX for moderated sessions

[benarent]

Description

While testing moderated sessions, I found the lack of feedback a tough UX. Sessions the require a use to join, look like they just hang.

What happened:

Teleport > Creating session with ID: 544c73f5-a56d-4160-8f70-4779e3ec433e...

Teleport > User ben-test joined the session.

What you expected to happen:
We should provide context for which roles need to join.

Creating session with uuid <example-uuid>...
This session requires moderator. Waiting for others to join:
- role: auditor-role x1

[xacrimon]

Hi, due to the RBAC model of moderated sessions, there's many possible combinations that could exist that would allow a session to start. Would broadcasting Teleport > Waiting for required participants... be sufficient or would we be better of making a list of sections, each containing a list of at least one item that must be satisfied. This feels very clunky to do in the terminal however and eats a lot of space.

For context, the RBAC model allows a session to start if at least one require policy is satisfied for every role that declares at least one such policy.

Such ux could look something like this:

Teleport > This session requires additional participants. Waiting for others to join:
  role1:
    - 2x auditor-role
    - 1x developer-role
  role 2:
    - 1x senior-developer-role 

[benarent]

Would broadcasting Teleport > Waiting for required participants... be sufficient

I think this will be a good place to start. But we might want to double check with @xinding33 . Could we also remove the space between the messages?

[xacrimon]

Yep, we can, as part of #10725 I'm making some changes to the UX, this proposal is also pending review from Xin. Let's move this discussion there.