List, identify, and kill agents

Current situation

Agents may die while still being registered. This situation (a stale agent) is handled by the agentchannel plugin, but over time this may lead to unnecessary resource usage.

Desired outcome

A way to list registered agents, and a way to deregister unwanted ones.

Analysis

The agentchannel API allows for listing registered agents and for agent deregistration. The endpoints are available, but there is no user-faced interface.

Solution

Add a get agents command and a delete agent _ command to opentf-ctl.