[Elastic Agent] Enrollment via proxy generates invalid fleet.yml (Agent won't start up correctly)

[urso]

For confirmed bugs, please report:

• Version: 7.14
• Operating System: Any

When enrolling Agent via proxy using --proxy-url the fleet.yml becomes invalid, as the URL is serialized as Object:

  proxy_url:
    scheme: http
    opaque: ""
    user: null
    host: 10.1.4.118:3128
    path: ""
    rawpath: ""
    forcequery: false
    rawquery: ""
    fragment: ""
    rawfragment: ""

The proxy_url must be a string.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)