Add forward proxy for verify auth token

[FabianZihlmann]

Add forward proxy for checking auth token. Optional proxy could be like http://proxy.domain.com:8080

Description

The http_client is now using a forward proxy if this is configured in the config.toml. So you're able to operate the syncstorage-rs server in a security environment where a server has no direct access to internet.
In this case the server is not able to verify the auth token from for example https://oauth.accounts.firefox.com and the you'll get the follwing error message in the log:
Error requesting remote jwks: error sending request for url (https://oauth.accounts.firefox.com/v1/jwks)

This resolves the follwing isseu #1964

Testing

Add the following line to config.toml
tokenserver.fxa_oauth_request_proxy = "http://proxy.domain.com:8080"

[chenba]

@FabianZihlmann I'm sorry about such a delayed response. The feature and implementation make sense. Can you add a unit test, and perhaps documentation to https://mozilla-services.github.io/syncstorage-rs/config.html? Thank you.

[FabianZihlmann]

Dear chenba
Thanks for your comment. Unfortunately, I'm not familiar how to make these unit tests. Do you might any other option?
Docs added, hope that is detailled enough? There was some mess up with not actual local code. I hope you can fix that. Sorry, I'm not really familiar with! (shame on me!)
Best regards
Fabian