Support for http proxy environment variables

[Gyunikuchan]

[REQUIRED] Step 2: Describe your environment

• Operating System version: node:8-slim docker container
• Library version: 6.1.0
• Firebase Product: Messaging

[REQUIRED] Step 3: Describe the problem

We're running our server behind a proxy and firebase-admin-node does not seem to respect the following environment variables:
http_proxy
HTTPS_PROXY
ALL_PROXY
NO_PROXY

We would like to request that the library's http client look out for these environment variables when making outgoing requests.

[Gyunikuchan]

Thinking it over, such a solution might introduce breaking changes to current users of the module...
A solution as suggested by #253 might be more relevant.

There are also ongoing discussions on the possibility of setting a global agent for node's core http module: nodejs/node#23281