Add proxy variables support to appcenter-file-upload-client-node

[russelarms]

This PR adds proxy environment variables support to appcenter-file-upload-client-node using proxy-agent package. It should fix the problem with uploading releases using a proxy.
This PR also bumps the package version. After the PR is reviewed and tested we should publish an update to npmjs and update the dependency of the appcenter-cli and use the new fetchWithOptions method instead of 'fetch'. There's a draft PR for it.

How to test:

1. Add an environment variable 'https_proxy' with proxy settings.

export https_proxy=http://xx.xx.xx.xx:port

2. cd appcenter-file-upload-client-node && npm pack
3. install the package from the previous step in appcenter-cli
4. change fetch calls with fetchWithOptions from "appcenter-file-upload-client-node". See this PR.
5. run upload release command. E.g. appcenter distribute release -f appRelease.apk -g publicGroupName

Tested:

• http and https proxy tested manually
• with npm test and npm test:ci
• ci tests

Related links:
Original issue: #982
The next PR: #1045
AB#81897

[annakocheshkova]

LG, not tested though.

[jp-andre]

Looks go to me, but there's no test verifying that an agent is set when the env var is set.

[russelarms]

Looks go to me, but there's no test verifying that an agent is set when the env var is set.

Added a simple test.