lib: support overriding http\s.globalAgent

Overriding `require('http[s]').globalAgent` is now respected by
consequent requests.

In order to achieve that, the following changes were made:

1. Implmentation in `http`: `module.exports.globalAgent` is now defined
through `Object.defineProperty`. Its getter and setter return \ set
`require('_http_agent').globalAgent`.

2. Implementation in `https`: the https `globalAgent` is not the same
as `_http_agent`, and is defined in `https` module itself. Therefore,
the fix here was to simply use `module.exports.globalAgent` to support
mutation.

3. According tests were added for both `http` and `https`, where in
both we create a server, set the default agent to a newly created
instance and make a request to that server. We then assert that the
given instance was actually used by inspecting its sockets property.

Fixes: https://github.com/nodejs/node/issues/23281

PR-URL: https://github.com/nodejs/node/pull/25170
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/https.js

@@ -296,7 +296,7 @@ function request(...args) {
     Object.assign(options, args.shift());
   }
 
-  options._defaultAgent = globalAgent;
+  options._defaultAgent = module.exports.globalAgent;
   args.unshift(options);
 
   return new ClientRequest(...args);