[v18][scopes] tctl scoped role assignment UX improvements

[nklaassen]

Backport #65628 to branch/v18

Manual Test Plan

Test Environment

A cluster on this branch with TELEPORT_UNSTABLE_SCOPES=yes

resources.yaml:

kind: scoped_role
metadata:
  name: eng-access
  revision: 17ce670f-f672-4a97-843d-dcc9ef80c65a
scope: /
spec:
  assignable_scopes:
  - /eng
version: v1
---
kind: scoped_role_assignment
sub_kind: dynamic
scope: /
spec:
  user: nic
  assignments:
    - role: eng-access
      scope: /eng
version: v1

Test Cases

• tctl create resources.yaml
• tctl get scoped_role_assignment lists the assignment
• tctl get scoped_role_assignment/dynamic/id works
• tctl get scoped_role_assignment/id fails with an actionable error
• tctl rm scoped_role_assignment/id fails with an actionable error
• tctl rm scoped_role_assignment/materialized/id fails with an informative error
• tctl rm scoped_role_assignment/unknown/id fails with BadParameter
• tctl rm scoped_role_assignment/dynamic/id works

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