feat: self-hosted agent get/list command

[lucaspin]

Adds a new get command to list/describe self-hosted agents:

» sem get agents
NAME       TYPE         STATE           AGE
my-agent-1 s1-aws-small waiting_for_job 12m
my-agent-2 s1-aws-small waiting_for_job 10m
my-agent-3 s1-aws-small running_job      8m
my-agent-3 s1-aws-large waiting_for_job  1m

» sem get agent my-agent-1
apiVersion: v1alpha
kind: SelfHostedAgent
metadata:
  name: my-agent-1
  type: s1-aws-small
  connected_at: 1686789756
  version: v2.2.6
  os: Ubuntu 22.04
  arch: x86_64
  hostname: agenthostname
  ip_address: 0.0.0.0
  pid: 0
status:
  state: waiting_for_job