enabling ProxyAgent take an instance of Agent as an option

[RishabhKodes]

In the updated constructor, the opts parameter is checked to see if it contains an agent property that is an instance of Agent. If it is, then the [kAgent] is set to the provided instance. Otherwise, a new Agent instance is created and assigned to the [kAgent].

fixes: #1962

[KhafraDev]

Can you add in a test?

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3606c35) 90.45% compared to head (d31d7ae) 90.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1998   +/-   ##
=======================================
  Coverage   90.45%   90.45%           
=======================================
  Files          71       71           
  Lines        6138     6138           
=======================================
  Hits         5552     5552           
  Misses        586      586           

Impacted Files	Coverage Δ
lib/proxy-agent.js	93.50% <100.00%> (ø)

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[RishabhKodes]

Can you add in a test?

Added the tests for both scenarios, take a look.

[mcollina]

Could you add docs for this change?

[ronag]

How is this different from #2003

[RishabhKodes]

How is this different from #2003

#2003 majorly targets the broader issue of successive requests to ProxyAgent getting blocked, thus introducing a new Client for solving the issue. This pr also does somewhat of the same thing enabling the user to create an instance of Agent (similar but less complex than creating a Client) inside the ProxyAgent.

This pr solves the inability to use ProxyAgent and any other Agent at the same time (example mentioned here, #1957).

[ronag]

Do you think we could use the same pattern with a factory?

[RishabhKodes]

Closing this as #2003 solves the issue in almost the same way (and that solution has been approved)