Add Agent / Install snippets to include proxy parameter if specified in the policy

[lucabelluccini]

Describe the feature:

• Add agent flow - include the proxy parameters to be used in curl/wget/Invoke-WebRequest if a Proxy is associated to the Source URI - also use the Source URI as base path for downloading the Elastic Agent
• Add agent flow - include the --proxy-url <url> parameter to be used in the elastic-agent enroll/install if a Proxy is associated to the Fleet Server URI of the policy
• Add Fleet Server flow - same of above for the download of the install binaries, plus include the proxy URL also for Fleet Server to reach Elasticsearch, if a Proxy is set up for Elasticsearch

Describe a specific use case for the feature:

When a user sets a Proxy to the Source URI, it's likely the Proxy has to be used both when installing and upgrading Elastic Agent.
The current installation snippet generated by Fleet UI is not providing any proxy parameter.

When a user sets a Proxy to the Fleet Server, it's likely the Proxy has to be used also when installing/enrolling the Elastic Agent as it has to reach the Fleet Server.

Comments:

This is applicable only for Elastic Agent where the Proxy settings can be specified from the Fleet UI.
If we're not able to generate the exact commands, maybe we could add a warning/popup near the snippet to suggest adding proxy parameters or env variables.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)