For app authors, create a HTTP proxy that calls httpGet

[paulproteus]

As an app author, I want to set a HTTP_PROXY environment variable for my app which would result in the app's HTTP communication being tunneled over the Sandstorm httpGet (deprecated) API.

I believe @jparyani has done some work toward this already. I believe the current question-mark is: how should we handle HTTPS? Typically the HTTP_PROXY environment variable results in the CONNECT verb being used, so (presumably) Sandstorm would need to create a HTTPS CA that the app trusts. One implementation strategy, for better or for worse, is:

• Each Sandstorm server generates a random key, and uses that when communicating with grains.
• Sandstorm exposes an API called getCACertificate() which when called, returns a PEM-formatted public key version of the certificate.
• Apps are expected to add that to their trust store somehow.

Other question:

• How should this interact with the powerbox?
• If we think this will never happen, then let's close this and point people at it when they ask.

Relevant discussion:

• https://groups.google.com/forum/#!topic/sandstorm-dev/4QGUA3mm4MU

Thanks to @ndarilek for requesting this issue be filed.

[paulproteus]

(Assigned myself by accident.)