Add `exec` param to `target` to run a process

[dherbst]

In the target section, add exec with a line to run something and capture the logs. The working directory should be relative to the ratproxy.yaml file, with a name exec-working-dir The logs can be seen in the admin listener.

Expected configuration:

listen: 0.0.0.0:1414
targets:
  - name: npm-run-vue-app
    target: http://localhost:5173
    path: /vue/
    exec: npm run dev
    exec-working-dir: ./web/vue-app
  - name: other-target