Add support to proxy outbound requests from Synapse in tests

[MadLittleMods]

Add support to proxy outbound requests from Synapse in tests. Adds new environment variables that can be used with the Docker image (SYNAPSE_HTTP_PROXY/SYNAPSE_HTTPS_PROXY/SYNAPSE_NO_PROXY)

Useful for things like the Secure Border Gateway

Why is this necessary?

You can already configure the HTTP_PROXY/HTTPS_PROXY environment variables to proxy outbound requests but setting this globally in the Docker image affects all processes which isn't always desirable or workable in the case where the proxy is running in the Docker image itself (because the Debian packages will fail to download because the proxy isn't up and running yet) . Adding Synapse specific environment variables (SYNAPSE_HTTP_PROXY/SYNAPSE_HTTPS_PROXY/SYNAPSE_NO_PROXY) makes things much more targetable.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

Thanks for the review @erikjohnston 🦞