[Elastic Agent] To adhere to a Proxy Auto Configuration (PAC) file installed on the same host

**Describe the enhancement:**
A lot of organizations would need their traffic to be sent to a proxy (say for security reasons). These users install PAC files that determine where to proxy to for a given url destination. Currently the Elastic Agent doesn't adhere to proxy URL set in a PAC file - it can read the proxy if set as an environment variable or passed in as an option when agent is being enrolled.

I believe this needs to be facilitated via another flag (similar to the proxy-url) that points to the PAC file to use. If this is defined then the environment variable and proxy-url are to be ignored.