Support proxy server configuration

Hi SWA-Team,

first! Great Tool! it works very well from my developer machine outside our company network.

Our CI/CD tool (jenkins) is part of the company network secured by a proxy server.

We are not able to fetch the `https://swalocaldeploy.azureedge.net/downloads/versions.json` file from the corporate network. (./src/core/deploy-client.ts:151)

**Describe the solution you'd like**
It would be great to have some proxy options to handle the outgoing requests. There is a nice package to add a proxy agent to `node-fetch`.

[https-proxy-agent](https://www.npmjs.com/package/https-proxy-agent)

Some cli parameters to setup the proxy configuration would be nice solution.

**Describe alternatives you've considered**

Does the docker-way do the trick? Does the docker image contain the `StaticSitesClient` and this step could skipped?

Thanks a lot for this great cli tool.

But for now, we cant use it