Allow for retrieval of known --to the orchestrator-- agents end execution environments

Context:

As of today, the retrival of the

• Agents is available through a GET call on /agents ex : curl -H "Authorization: Bearer " http(s)://orechestrator.server:{agentport}/agents
• To the best of my knowledge there is no way of getting informations about ssh execution environements execpt by looking at the orchestrator configuration (ex : docker exec orchestrator "cat /etc/squashtf/pools.yaml"

As a user of opentf-ctl I should be able to retrieve this infos via the CLI.

Proposal

• Add a command ** opentf-ctl get agents ** to retrive list of known agents
• Add a command ** opent-ctl get ssh-exececution-envs" to retrieve ssh execution environements
• (OPT.) Add a command ** opent-ctl get execution-environements" to retrieve both. In fact this may be the only command one needs using flags --agents or --ssh-exec-env to filter the results accordingly

Impact

As, to the best of my knowledge, there are no endpoints on the orchestrator for the retrieval of ssh execution environment, such an endpoint shall be created also.