Proxy Support

**Describe the bug**
When trying to install and run behind a corporate proxy solution (MITM) the extension will time-out after some while with the following message

> Unable to load Lambda Functions, click here to retry

**To Reproduce**
1. Sit behind a proxy/firewall with working `HTTP_PROXY` and `HTTPS_PROXY` config.
2. Install according to README.md
3. Run Connect to AWS:

#### Select your profile in Visual Studio Code

1. Launch Visual Studio Code.
2. Select `View > Command Palette...` and search for `AWS`.
3. Select `AWS: Connect to AWS`

**Expected behavior**
Something should load ...

**Desktop (please complete the following information):**
- OS: Linux
- Visual Studio Code Version: 1.30.1
- AWS Toolkit for Visual Studio Code Version: 0.0.1

**Additional context**
There is a experimental setting in VS Code
`"http.proxySupport": "on"`
This does not work.

Reproduce the failure using the README.md installation steps, Linux, and working HTTP_PROXY and HTTPS_PROXY settings, then run VS Code's AWS: Connect to AWS command. Inspect how the extension handles the proxy while loading Lambda Functions; done means the functions load successfully behind the corporate MITM proxy instead of timing out.

Written by the indexing model from the issue text.