Add support for proxying requests in apiv2 (#2996)

* add support for http_proxy variables

* use extends in tsconfigs

* add types for the proxy module

* -only

* simplify tsconfig by extending the base

* add proxy information to the readme

* add changelog entry

Author: bkendall

Diff for: CHANGELOG.md

@@ -1,5 +1,6 @@
-- Allow more than 100 concurrent connections to the Realtime Database emulator.
+- Allows more than 100 concurrent connections to the Realtime Database emulator.
 - Fixes incorrect `databaseURL` inside the Cloud Functions emulator for new projects (#2965).
 - Fixes function URLs when emulating namespaced/grouped Cloud Functions (#2966).
-- Support using `firebase-bolt` installed in local `node_modules` (#767).
+- Supports using `firebase-bolt` installed in local `node_modules` (#767).
 - Fixes issue where Auth triggers were not disabled when background trigges were disabled.
+- Re-adds support for proxying HTTP requests in a new underlying library (#2927).

Diff for: README.md

@@ -185,6 +185,12 @@ that we pass into the emulator through `GOOGLE_APPLICATION_CREDENTIALS`. We **on
 if you have not already set the `GOOGLE_APPLICATION_CREDENTIALS` environment variable
 yourself.
 
+## Using behind a proxy
+
+The CLI supports HTTP(S) proxies via environment variables. To use a proxy, set the `HTTPS_PROXY`
+or `HTTP_PROXY` value in your environment to the URL of your proxy (e.g.
+`HTTP_PROXY=http://127.0.0.1:12345`).
+
 ## Using with CI Systems
 
 The Firebase CLI requires a browser to complete authentication, but is fully